Do you like any blocks that use a 3 1/2" (3" finished) square in them? If so, you could substitute the signature blocks for one or more of squares in the block. You could even do these as a sampler of sorts since you like quilts which have different blocks in them.
ETA: Really, you could look at any 9-patch or any 16-patch or any 4-patch block and easily adjust the pattern so that you're using a 3" grid.