I have learned a few things from my limited blueberry as bonsai experience, and from pruning a few thousand blueberry bushes, but I am only a couple years into learning about blueberries, so my knowledge is more ''book and internet and Ag Extension agent knowledge" rather than hands on real world experience.
The normal habit of a blueberry is that they are very long lived shrubs, bushes a century old are still productive in commercial plantings. Their root system is the long lived part of the shrub, they routinely send up new sprouts, which usually emerge as long unbranched whips. The terminal bud of every whip and secondary branch tends to die in winter, so each years extension tends to be from either the root system or from secondary branching of the branch you are looking at. Year one is a whip with no branches. Year 2 and 3 the second and tertiary branches are formed. At the end of the third growing season, on robust branches, the terminal bud will become a flower bud, first blooming and fruit can happen as early as the 4th growing season. Each year afterwards, the branches will continue to ramify, and the terminal buds where the branch is vigorous and has good light, will form flower buds and fruit. As the levels of ramification increase, the total number of flowers continues to increase but the total weight of the larger number of berries remains constant, meaning the berries get smaller, more in proportion for bonsai, but bad for the commercial fruit market. At about 10 years and up, the total yield by weight, of fruit from a ''cane'' or branch coming from the roots begins to decline, and eventually, usually by about year 20, the root system itself withdraws support for the "cane'', and that branch dies. So when looking at a century old blueberry bush that has not been pruned by a human, none of the above ground wood will be much older than 20 to 30 years old. The bush ''self prunes'' to eliminate older wood.
I used quotes around cane, as the stem from the rhizome is technically not a cane, blueberry stems are true wood, they could be considered trunks, or sub-trunks, or branches in the same sense as a branch or multiple trunks of a maple are true wood. Blackberries and other brambles have true canes which have a central core of pith, rather than true wood.
For fruit production, pruning has the goal of eliminating wood over 10 years old, and encourages new shoots from the roots to replace the older wood that is removed. An interesting aside, most highbush blueberries are self fertile, and don't need cross pollination. Most northern low bush types require pollen from a different clone to get fruit. Solitary orchard bees and bumble bees are the most efficient pollinators, the honey bee usually has to visit a blueberry blossom more than once to get pollination. Blueberries bloom while weather is too cool for honey bees to be a peak activity, so the native solitary be species are the best for blueberries.
For bonsai purposes, a shoot can be encouraged to last 20 to 30 years without much trouble, and a good fertilizer program will help it to possibly last longer. But do plan on letting the occasional new shoot survive, to eventually replace the oldest ''trunk''. Every year, multiplee new shoots should emerge if the bush is healthy, remove new shoots from the root system, in order to preserve the oldest shoot. Blueberry branches, once they have more than 3 levels of ramification have a bad habit of ''self pruning'', withdrawing support from the ramified secondary branch in favor of a young, unbranched epicormic bud on the primary ''cane''. As bonsai you will have to keep an eye out for these, and remove them before their second growing season, or the stem will withdraw support from nearby branches that have multiple levels of ramification. A middle of the growing season pruning to remove these juvenile unbranched shoots should be good enough, I do mine no later than the middle of August. This way any new growth stimulated by pruning will have time to harden off before winter. A good time to prune blueberries and not affect their winter hardiness is after leaves have dropped in autumn, through winter and up until new growth resumes in spring. So in my area, the two time for pruning are winter dormancy and July into early August. Wiring is best done in autumn or winter. Bark is thin, like on azaleas, so wire should be checked often and removed the minute it begins to cut in. Bark of young branches tends to be a beautiful rusty red-purple. Older bark is gray, but thin with some flakiness. Autumn leave color is in various shades of yellow, red and purple, and can be quite nice. The white bell shaped flowers in spring are delightful, and the blue fruit is tasty.
This all sounds complicated, but once you get into it, it is less complicated than training an azalea. (a distant cousin of blueberry).
Note: the habit of a blueberry to have the terminal bud abort, or make flowers, means that primary whip, from the base of the roots will not add additional height the subsequent seasons from the main stem. Additional height will come from a side bud that bends itself, or is wired by you into position to continue to gain height. You currently have 3 trunks with the oldest and thickest being the shortest. You will have to keep the more slender trunks in check, and encourage, by wiring, the thick trunk to grow to be taller than the other two, or your sense of perspective will be lost.
I think Sorce's virtual can work, as can your original plan. If it were mine, I would keep only a single, probably the oldest trunk, eliminate the rest, then when new shoots emerge the next growing season, keep the one that is in the "right spot, with the correct angle" and eliminate the rest. Do it as a "mother daughter" twin trunk until year 5 to year 10, then allow a third new stem to develop, and train it as a 3 trunk connected root clump. Because new shoots will have zero ramification, and be sparse for several years, a blueberry bonsai will only be in "exhibition form" for maybe 2 years out of every 10, but heck, this is true of most trees. Most of the time, they will be pleasant to look at on the bench at home.
Remember, as a vague guideline, twin trunk designs the secondary tree is either one third the height of the main tree or two thirds the height of the main tree. A twin trunk where the secondary tree is approximately half the height of the main tree tends to look awkward. In the 3 trunk design the secondary is two thirds, and the third is one third the height of the main tree. These are suggested guides, as they have been proven to work in the aesthetic of Japanese bonsai, you can do what you like and if the tree in front of you is attractive but doesn't conform, that is perfectly acceptable.