Do anyone think that a bishop and a knight is worth a rook at the begining of a game.

Historical chess wisdom has considered Bishop and Knight equals the Rook... they are probably  better depending on the position.  A Knight in the center on the sixth rank is probably worth more than a Rook.  If you play the Sicilian Dragon then the exchange sacrifice of the Rook for the Knight (and a pawn) is generally acceptable.

Tarrasch argued that the difference between the Rook and a minor piece is not a winning advantage.  Possibly that is if there is not a second advantage (or imbalance).

Bishop + Knight > Rook (at the beginning of the game)

Bishop + Knight > Rook (endgame)

Bishop + Knight >= Rook + Pawn (greater than or equal to, when the rook has a pawn)

Bishop + Knight < Rook + 2 Pawns

In the Dragon Sicilian:

Rook = Knight + Pawn

well for me yes!

Part of it is judging the position and possibilities.  For instance, if you can gain a lasting initiative by giving up the Rook for a minor piece then it may be worth it.  Typically the two minor pieces are better because they have the potential to cover more ground and control more space than the lone Rook.

Another way to understand this is "piece value."  Traditionally the Rook is worth 5 points while the Knight is worth 3 (or 3.5) and the Bishop is worth 3 (or 3.5).  Based strictly on value the Knight and Bishop are worth more than the lone Rook.

However, other things factor in to the game (initiative, pawn structure, superior minor piece, etc...) and you must understand what risks are involved in the transaction.

A book that might help you get a handle on this would be Jeremy Silman's "How to Reasses your chess."  It's not infalable, but it's very good for understanding.

Ajfonty said it right

Of course it depends on the specific position -- but I'd usually rather have the bishop+knight.

Bishop+knight, I reckon, need not be sacrificed for a Rook. But, at times, it pays to sacrifice Bishop+Knight for a Rook+Pawn.