Triple SLI on ASRock Extreme6?

Motherboards are my worst part of a computer as far as knowledge goes, so I apologize if I sound dumb. From what I can see, the Extreme 6 has 3 PCIE slots. However, further research has shown 2 PCIE 3.0 x16, after which it says (x16/0 or x8/x8), and 1 PCIE 2.0 x16, after which it says (@x4). From what I see, it looks like the PCIE 3.0s will use x6 with 2 cards, but the 2.0 will use x4. Does this make it impossible to use 3 way SLI? And if 3 way SLI is possible, will the x4 affect the x8 of the other two cards? Also, I have no idea what x16 x8 and x4 are. Again, I know nothing about motherboards, sorry! Thanks.