I need to get my hands on 3. LM1 and LM2 are both great for different reasons.
Generally I liked LM1 more despite feeling that LM2 was a better game functionally, but the separate stages of LM2 let it down and I felt LM1 had a better atmosphere, which gave a more lasting impact in my mind.
While I cannot comment on LM3, the studio seem to have learned their lessons from the previous game and they've been given a massive budget for it. It does look absolutely amazing and I can't wait to have my hands on it.