I am working on an Alder too. Not commonly used as bonsai but we get plenty of volunteers around the yard so I decided to try one. So far I have found that they take well to deadwood carving, and will back bud after a chop. Because they grow like weeds, I have been using clip & grow to develop ramification (still in early stages) so I haven't tried wiring it yet. It's too early to tell how much the leaves will reduce, last season mine were probably 1/2 the size of the local alder leaves but still too big for mine at 8".
With yours I like the triple trunk. I'd probably chop all 3 trunks at about 1/2 and hope for a new leader on each, then start over with branch development.
Do you know what kind of alder it is? Here we have red alder, Alnus rubra.
