yep that's it. For a 3x3 you'd need a few sheets as you'd use one the top, one for the bottom and one for the back.... then you'd need sides.
Unless you fiddled the sizes a tiny bit.
6 x 3 and a 2 x 3. Plus plinths for runners.
6 x 3 and a 2 x 3
6 x 3 and two 1 x 3.
so you've two 6x3's for the top and bottom and an almost 6x3 for the back.
A 2 x 3 and a 1 x 3 for each of the sides to give you a 3 x 3 side. You'd also have enough left overs to make a centre support for the front (if you think you might need one) and some panels to cover the joins on the sides.
you'd need to work the sizing out though as you'll lose 3mm off each cut, so you won't get an exact 6ft and 2ft from an 8ft sheet.