Why doesn't the Tu-154's center engine have a thrust reverser?
Several I guess:


There is no way to store them on the middle one in a way not increasing drag during flight.
In any possible configuration they would blow hot exhaust always against some body part, thus making it rather inefficient.


But maybe most important:


Two reversed engines already produce more than enough breaking power, no need to spend money on another. In fact, even one would be good for all purpose, but asymmetric force isn't a good idea either :))