Have you read this on the Thru-Hiker website?
I haven't sewn these yet but they look interesting.
>>know if I use all 6 pieces to make one mitt or if I just use 3<<
From what I see and read you'll use 3 patterned pieces along with two of the elastic pieces per mitten.
Think of it like this; you have 1 piece to cover the back of your hand and wrist, 1 slightly smaller piece that covers your palm and the inside of your hand/thumb and 1 even smaller piece that covers the back of your thumb and the inside of your wrist on "one" of your hands.
I hope this clear as mud description helps. ;-)