Tri band = 2.4ghz + 5GHZ + 5GHz (Most common for today's market. I would actually expect this to change if WIFI6e becomes a thing)
So it's just an additional radio in the same AP. They are really meant for dense deployments where there are a ton of clients. (Think board room, auditorium, etc) In a house it will probably have the opposite affect you want. Since all 3 radios are in exactly the same spot, the coverage isn't going to be any different. If you only have 4 or 5 devices on wifi, you'd have to have some serious bandwidth needs for it to matter, and if you need that much you're better off hard wiring them anyway.
Think of it as 3 APs just bolted together, because that's what it is. One client can only ever connect to a single AP at a time. So the max speed you'll ever get is what one AP can provide. The most common configuration today is going to be 2 x 2:2 @80mhz which gives around 867mbps theoretical (~520mbps actual in half duplex) with full bars. It doesn't matter how many APs you bolt together, this isn't going to change. The one time where it may matter is if your dedicating say 1 radio to a single device. Then in theory that device will have dedicated bandwidth to it, and stuff that other clients are doing won't affect it. But that's like saying DSL or fiber is better than cable because it's not shared. Everything is shared at some point, so there will always be a bottleneck. (And in the case of wifi neighboring devices that are sharing that "dedicated" channel can still destroy your speeds even if you only have a single device because of the way wifi shares airtime)