7 inch x 1.41 = 9.87 + 1.25 = 11.12
I'd round it up to a 12 inch block and cut it on the diagonal both ways and you will have 4 triangles. Sew one triangle to each corner.
Square up the block and you are done.
I'm not sure what you mean when you said you could get one but need two more. You need 4 for each 7 inch block.