there's a few ways you could go about this...
1) the easiest would be to put a border around the block to make it a 12 inch block. since this is going to be a sampler quilt you don't need to have the corner patches match up to form a secondary pattern.
2) the next option would be to make the corner patches 5 inches instead of 3 inches and then you could leave the paper piece pattern the way it is and end up with a 12 inch. this kinda throws off the ratio of the sections to each other but it is an easy change to make.
3) and then the next thing you can do is increase the paper piece template to 3 inches on a copy machine and then then corner patches would be 4.5 inches and then you'll maintain the ratio of the sections to each other.