It's generally a win for the three bishops. However, it can be a rather long process.
The tablebase I consulted says that in this position with White to move, the best move for White is Kd3, which is a win for Black with DTZ of 87.
[FEN "8/2kb4/8/8/8/2K1b2b/8/3R4 w - - 4 3"]
In fact, in some positions, the 50 move rule will trigger, which could result in a draw with best play. Here, Kd5 results in a DTZ of 101:
[FEN "k7/3b4/8/8/2K5/7b/5b2/1R6 w - - 1 1"]