Is it a good to trade a rook for a knight and a bishop. The computer used it against me in hard mode, but I don't know if I would do it myself in a game. Thoughts?

Yes, in most positions 2 minor pieces are stronger than one rook .

 

Position, position and position.

 

But usually one if you trade your rook for an opponent's bishop and knight, I'd take the two minor pieces in general. But usually there's a pawn that comes with the rook, so if you add the pawn in the bargain it really depends on the position.

I would almost always give up a rook to win a bishop and knight.

In the case of when your opponent is castled and you take twice on f2 or f7 it usually favors the side getting the two minor pieces for the rook and pawn.

Here's the points that pieces are worth

pawn=1, knight/bishop=3 rook=5 queen=9

 2 knights are better than a rook in early game and middle game but it is better to have a rook in end game only if there are not a lot of pawns

 2 knights are better than a rook in early game and middle game but it is better to have a rook in end game only if there are not a lot of pawns

 

In the case of Rook vs Bishop & Knight with equal pawns though, even in the endgame the minor pieces usually do better. Flear did a survey in "Practical Endgame Play" showing the rook winning 0, drawing 10, and losing 22 games.

Two minor pieces are better normally. They can attack the same square twice. With a pawn as well they are better.

In general, assuming all things are equal positionally the answer is yes, and it comes down to simple math. Excluding the pawns and kings, both players have 8 pieces. To give up one rook to gain two minor pieces is well worth it. Now you only need trade down equally until you have one piece left on the board, and your opponent has none. Though don't do this with a queen.

I'm mixed.  I feel it all depends on the position.  Often times there's another factor in these trades...like strong attack/what have you. 

The one game I did trade bishop and knight for rook, I ended up winning!
(not saying that's a good enough reason why to do it)

Thank you all for your responses. I will definitely use them when playing. This is something I wanted to know, so many thanks :D