I have a pawnless game with a rook, knight, and king against just a king. Can I checkmate from this position?
[FEN "8/8/8/8/1n6/8/2rk4/1K6 w - - 0 1"]
Yes, since K+R is an easy beginner checkmate. In fact, the Knight will get in the way. The only value of the Knight will be to get White into Zugzwang quicker (easier?), which helps Black push the king to an edge.
To answer the specific question, from above, assuming Black to move:
1... Kc3
2. Ka1 Rc1#
Assuming White to move:
1. Ka1 Kc3
2. Kb1 Kb3
3. Ka1 Rc1#
As the first responder indicated, K & R vs K is a basic checkmate for the stronger force. All he must do is drive the Rook to the edge using the coordinated K & R and administer the checkmate there. Having additional material, while redundant, can only simplify matters for you.
[FEN "7k/7R/5N2/8/8/8/8/4K3 b - - 1 0"]
The knight and rook need no help to deliver checkmate.