best now motherboards am3+ for overclock fx-6300 and 3 x MSI GeForce GTX 660 TwinFrozr 2GB

The Crosshair is obviously the better and higher end board, but the Sabertooth R2.0 has almost the same features and is considerably less money. It also supports 3 way and Quad SLI. I have this board and would probably not pay the difference for the Crosshair.
 
Actually, the GTX 660 does not support three way sli, except for apparently one specific Nvidia reference model. Do you already have three cards? If not, I wouldn't go that way at all. It would make a lot more sense to just get one more powerful card. What country are you in and what websites do you prefer to buy from?
 
I don't see very many people from Poland here on the forum. Welcome to the site. I have many Polish relatives of the name Wojdyla, some here, some there. Anyhow, the 660 seems to have very good dual card performance so I think you'll be happy, all things considered.