Are the moves,
1. a8=R Kb1
2. Rxa3 Kc1
3. Ra1#
or
1. a8=R Ka2
2. Kb2 Ka1
3. Rxa3#
1. a8=R a2
2. Rh8 Kb1
3. Ra8#
Nice spot with the rook promotion, would be quite easy to fall into the stalemate trap if your rushing.