This should be simple but I am not sure. I have three lights in one fixture, sofa1, sofa2, sofa3. If I want to treat them as one, do I put them into a scene or a room? In either case I don’t necessarily see how I get the toggling. Or do I haver to have a “button” for on and another for off?
Also my ST is connected to Alexa, so I would like to say “Alexa Turn Sofa On” and “Alexa Turn Sofa Off” - should they be in a scene or a room?
Neither.
A scene is a single snapshot in time. It is neither on nor off. It definitely doesn’t toggle. Remember that a scene might have the nightlight on and the overhead light off. So saying either “on“ or “off“ for that scene could be confusing. 
(Most people say they “activate“ the scene.)
So you could have scene A wHich has all three lights on, and Scene B which has all three lights off. But you have to issue instructions for each separately and you have to remember which is which. 
“Rooms” in smartthings are just used to organize the UI in the app visually while you are working with it. They don’t actually affect the devices or group them together for the purposes of commands.
Instead, grouping lights depends on which version of the app you are using, the classic app or the new “smartthings (Samsung connect)“
In the classic app, normally you would create a virtual switch and then have the individual lights turn on when that virtual switch turns on, and turn off when that virtual switch turns off. That makes it really easy to either use a group or use each one individually, both in smartthings and with echo. There is a how to article in the community created wiki on doing that:
http://thingsthataresmart.wiki/index.php?title=How_to_Group_Lights_Together
I believe there’s an official way to do it without the virtual switch in the new app, but to be honest I don’t know what it is. Hopefully someone will come along who does know.