DDR3 does not dictate that a motherboard will have a specific number of slots.
The socket 1366 / Bloomfield / X58 motherboards are triple channel - which means they are intended to work best when you put in 3 or 6 sticks of RAM. And they use DDR3 memory. You can buy DDR3 RAM in sets of 3.
But other motherboards that also use DDR3 typically have 4, or maybe 2, slots for RAM. For these boards, you would buy DDR3 RAM in sets of 2.