Solution to helpmate number three
The idea of this problem is selective promotion followed by self-blocking using the promoted piece. In the solution promotion to only the specified piece will work. Alternatives give check (if promotion to a Q or B) or self-blocking cannot be carried out as a flight square for the black king remains so that white cannot give mate (if promotion to a Kt).
Solution (black moving first):
Black White
1.f1=R Bc1
2.Rd1 Re3 mate.