For GA that's Terminal 3. At Terminal 2 (if flying SQ, and several other airlines IIRC) you can use Fast Track immigration if flying Business. Instead of walking to the right past all the check-in counters to immigration (and then doubling back airside past all the shops to the lounges), you can clear immigration to the left of the SQ counter, go through a magic door and end up airside in front of the SQ (and other) lounges.