This thread talks about getting credit for two tickets on the same flight. A brief summary is that, if the flight is a code-share, you might be able to get credit for one seat in each of the two programs by buying one seat with each FF number, getting two boarding passes, and checking in with both (so both BPs are collected for credit). You could either say you got the second seat to be comfortable, in which case they ought to be adjacent, or put your cello in it.
Three might be tricky, though flights operated by one airline and code-shared by two or more others exist. Maybe if you carry two cellos?