We are traveling from Heathrow Airport Terminal 2. We would like to visit a lounge in Terminal 1 or 3 before we fly. Is it possible to check into T2 and then move between terminals before we fly?
Normally, it is not possible to move freely between terminals at Heathrow, unless you book a shopping transfer. You need to check in and clear security in your departure terminal, and then otherwise need to stay there. On arrival, if you're transferring to another terminal, you do that before clearing security, so you can't enter the other terminal
However... Terminals 1 and 2 are special. They currently share quite a few facilities, including sharing a pier. That will continue for another year or so, until they finally close down what's left of Terminal 1. (Terminal 2 will then be expanded over the site of the current Terminal 1)
You can find the details on how to walk between the two in this thread on FlyerTalk, expect to allow 10-15 minutes. (You basically need to walk to the shared pier, then follow the signs to whichever main terminal you want to be in)
That said, the lounges in Terminal 2 are brand new and miles miles better than anything in Terminal 1, so you'd really be much better staying in T2 and visiting the lounges there!