But how will it work? Will there be 3 buttons?
We don't know yet. It will likely be an option you set for that wallet specifically (we have multiwallet, so you can have a wallet only for legacy addresses and one only for bech32). I'm not sure if P2SH nested addresses will be supported as that is mostly a shim for people to be able to use segwit while native segwit addresses (bech32) was still being worked on.
I see... but isn't that a bit of a mess? If I want to run a full node, now I need to run 2 different folders with 2 different software with 2 different clients just to go between formats?
Wouldn't it be better to have it all inside the same software and be able to select what you want or maybe that would make newbies too confused and it's better that newbies use segwit by default (since they don't know anything about this anyway and sometimes excess of options make newbies stressed)? But then again, advanced users like myself would suffer from having to run 2 softwares..