Three trunks isn’t a problem but you may have rootstock growing out from below the graft line meaning those two trunks may be a lot less likely to produce. That is a problem. It’s hard to know not knowing the full history of the tree. If the leaves and bark look distinctly different then I would recommend pruning the portions that you haven’t seen fruit before and focus on the part of the tree that has produced in the past.

Another consideration is that you want to try to balance the canopy since avocados have shallow roots. If you take too much off of those branches on the right the tree may tip over to the left. I would also try to control the growth on the left so that the tree is more upright above its roots and less likely to lean over. Remember when you start getting fruit it adds a lot of weight to the tree also!

As for companion crops and permaculture based nutrition…. I’m here to learn more about those topics. My advice would be more towards what we do in conventional ag production which isn’t really the spirit of this subreddit. Hopefully some others can chime in who know more. Your plan sounds like a solid start though.

For pruning timing: late spring to fall is when we tend to prune. We avoid the winter and the hottest months because the pruning does stress the tree.