I think your problem stems from the size of the quilt wanted versus the width of the fabric used. Most quilt fabric is approximately 42 inches wide. But a twin sized quilt would be about 65 inches wide by about 85 inches long. If you want 3 fabrics, I would then cut the main fabric piece 40 inches wide by 60 inches long. I would then take the second fabric and cut two pieces 13 wide by 60 long and two pieces 13 wide by 40 long. Then I would take the third fabric and cut the cornerstones 13 by 13 inches. These are all approximations and I didn't bother adding seam allowances but it should give you an idea of what you need.
Fabric requirement would be main colour 1 3/4 yards, second colour 2 1/4 yards and cornerstone colour 3/4 yard. These are all rough estimates and do not include binding.
Hope this helps a bit.
Okay, just saw that Katier has given you a very good visual. Much easier to understand than my instructions.