Prism99

If you are using 3.5" wide strips and 3 strips per block, I would strip piece the 3 strips together and then cut them at 9.5" to make 9.5" square blocks. When sewn together, finished block size will be 9". (Note: it's a good idea to actually measure your width after the 3 strips are sewn together and cut the blocks at *your* width -- which may be slightly smaller or larger than calculated measurements. As long as they are square, any size block is fine. For this reason and for efficiency, this is why I would not pre-cut all the strip lengths but would wait to cut until you have strip sets sewn.)

Assuming you mean a 3" finished outer border and 2" finished inner border, this means that the borders will take off 10" from the finished dimensions of length and width. If you want your quilt to be 90x100, then the inner portion of blocks would need to measure 80x90. This would be about 9 blocks across (9x9"=81") and 10 blocks down (10x9"=90").

Just be aware that if you use a cotton batting, you can expect about 3% shrinkage when the quilt is washed. This would make after-wash dimensions about 88"x95". If you want the post-washed quilt to be larger than that, you might want to add an extra block in both the width and length of the middle portion, or make the borders a little wider, or add a third border.

Caution: Check all my figures, as it is late and I could have made a mistake. I won't check my calculations until tomorrow!