try using a sheet of graph paper & sketching out your layout-with your 24" blocks---see if you find it pleasing & balanced...3 (24") blocks in a twin sized quilt doesn't seem as if it would leave much room for any other blocks-but it would probably depend on the blocks themselves...start with graph paper & play with it til you get what pleases you & have fun-
you can print off free sheets of graph paper from the internet.