The official answer is that YOU MUST READ THE WHOLE FOLD OUT LABEL of your Bayer 3 in 1, and the Neem. (the full label is book length) The label MUST list the pest you are trying to kill, or assume it will not work on your pest. So READ, because not all formulations of a pesticide will do the same in every location. Also READ to make sure it lists your plants as being safe for the pesticide to be used on. And finally, READ the SAFETY PRECAUTIONS.
Bayer has different formulations for California than they have for the rest of the country. It is state by state where approval for different products to be sold. My vision of absolute hell would be, I'm stuck as a compliance officer for State regulations for a pesticide company. Michigan approves pesticides for use on a county by county basis. The regulations are insane.
I do know that in all probability the Bayer 3 in 1 is a good choice to kill aphids. If the product has not been over-used in your area, you probably won't run into a resistant strain of aphids. Malathion is another old fashioned, but fairly effective poison for aphids.
Neem, because it is a plant extract, falls into a different category, and the label might not be as helpful. Neem does not outright kill insects. Its main effect is that it is an antifeedant. It causes the insects that come in contact to loose their appetite, and slowly starve. Neem also has some mild anti-fungal effects. It is a good product, but not 100% effective, I would alternate, 3 in 1 first week, to knock down populations, then a week later follow with Neem, the a week later another application of 3 in 1, then Neem. Keep the cycle going, note when you last saw a living insect, then one more application, then you are done.
Note, don't just spray your Chojubai. If one tree has aphids, there are probably low level populations on every green plant within 25 feet of the Chojubai. Spray your whole collection, and any close by garden beds.
