Why does my '93 Mitsubishi Diamante LS 3.0L DOHC have trouble accelerating?

Check the cam belt timing marks. It sounds like you're off a couple teeth. You don't have 4 camshafts, it's a SOHC, one cam per cylinder head. If some how a DOHC motor was modified to fit you will have more problems like the wrong ECU, the wrong wiring harness etc.