The thing to keep in mind regarding Mesh networks is that, if you can hardwire them then they aren't really necessary. Mesh is a wireless only consideration as it is primarily just "intelligent" routing between multiple wireless AP's. Sounds like the two units you already have are already wired. If you can wire the proposed third location that would be great. If not then maybe Mesh can help. Given your current hardware tops out at 802.11N, I'm betting you'll be pleased with just about any decent 802.11AC.The idea I've had kicking around in my head for about three years now is to get a 3-pack mesh system. Main / 1st unit would be in the basement homerun replaceing the current AEBS. 2nd unit would be in bonus room where AX is. 3rd unit would in theater or in master bedroom (if in MB, then 2nd unit might get moved back into spare room to better serve that space. I could possibly add a fourth unit to be: Homerun, Theater, Guest Bed, Master.
If you decide to go the Ruckus route [I would if I were starting from scratch] then @Dave Upton has already given you better direction than I could.