The actual forum will most likely be 1 forum for all sites, it's just easier to manage that way, and also will have the benefit of combining traffic into a single place. Hopefully it will be easier to promote that way.
I've barely started yet but I do have a dev environment ready to rock and roll. This will be a big deployment as I will need to convert all the posts from the 3 forums into a single forum. This will be more involved than simply moving data to a new database with different field names as I can't keep the same post/topic etc IDs.
I have not fully decided what the plan is but I have some general idea. I'm also an uptime evangelist, I've seen places take on a project like this but they take down the forum for DAYS! Nope nope nope nope. I will not do that (even though two of the forums are pretty much broken at this point due to a php update) The whole process will happen locally on my dev server, be pushed to test for further testing, then uploaded. The DB conversion will probably be done offline as I have that luxery given the forums don't get much activity.
One of the main reasons the forums died off as well was spammers. I had to turn off registrations, it just got insane. This new revamp will incorporate a sign up page that is separate from the forum, and will have some anti spam/bot measures. Still need to research that, but I think just having a separate form alone may help things. Also having 1 forum to moderate will help. Can't stop all spammers but can at least try to reduce them.

This should be a fun ride, I have not updated any of my sites in FOREVER. Will actually be nice to have something more modern going on again, and hopefully I can add more articles and other content and get some traffic flowing again.
I may even convert this blog over to a self hosted solution. TBH this blog was quickly thought out originally and not really used much, I'm normally not a fan of hosting stuff in a system I don't control.