Hmm, in all three he resolves the case with the help of a woman (murderer's accomplice in the first, murderer's daughter in the second, and victim's wife in the third). Yeah, it's not that is it. Perhaps a clue is needed.

Clue please.

It's subtle. Look for it at/near the beginning of each episode. Not sure how else to offer clues without giving too much away.

In all three, the murderer almost leaves something behind at the scene of the crime: the handkerchief on the phone, the briefcase, and the lighter.

BINGO!!! Nice job, TR.