How many memory sticks i can use ?

I want to build a new system and i will buy a motherboard with Z68 chipset.
I have 6 Gb Corsair DDR3 XMS3 1600 Mhz (3 x 2Gb sticks), i know the Z68 is dual channel.
I can use the 3 sticks of memory or i can only use 2 sticks?
I have to buy one memory stick more or what can i do?

You can use 3 sticks, but I believe, 2 will be in dual channel and the other in single. Not optimal. Also, the 1.65 volts can be a bit finicky with P67/Z68. Personally, I would sell what you have and get a proper kit for Z68.