Sonos would likely recommend you purchase both, but note the Sub-Mini is S2 compatible only whereas the Play:3 is both S1/S2 compatible - I, as another Sonos user, would suggest to go with another Play:3 to begin with, as their price can pretty reasonable in the ‘used’ market and their bass frequency is pretty reasonable anyway and a stereo pair is great for music etc.

You can always add the Sub-Mini later on, if you are using the S2 App, but the older Play:3’s may not be around for as long as the much newer Sub-Mini product.

I’m happy with a stereo pair of Play 3s in a good sized dining room.

I wouldn’t say no to a mini-sub but I don’t feel the need for one in there either.