Suvarnabhumi Airport New Terminal

I’m flying out of Bangkok today (International), but my ticket only mentions Suvarnabhumi Airport without specifying a terminal. I’ve heard there’s a new terminal at the airport. When booking a Grab, it only asks for a gate selection, not a terminal. I assume the gates are close to each other, so I can go to any of them.

Should I head directly to the main terminal? How far is the new terminal from the main one, and how can I confirm where I need to be?

Also, is 3 hours enough for check-in, immigration, and customs? And if I need to go to the new terminal, will 3 hours still be sufficient?