The FAA doesn't like it for Part 121 ops. They require extra Proficiency checks...
True now, however, in the past I personally knew two guys who held three types on license, current and line qualified.
They both were at TWA, and were triple line qualified...B747, B767, L1011.
Concurrently.
Extra sim checks?
Yes.
However these folks were very switched on, and enjoyed the challange.
Both Captains had their favorite, of course.
It was....?
TriStar.
No surprise, either. It simply is a better designed airplane.