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.