If the others are fine, you could put an amp at the split where the most signal loss occurs. But then, you could probably have it further upstream as well w/o overloading anything and spreading the gain across all lines. You can always try using coax couplers to take all the other TV's out of the mix and see if the troubled unit tunes better. There's still some loss at each coupler. In the end, I think you're on the right track, as the split three ways is impacting your viewing.
(did I just repeat what you asked?!)
