If your ruler does not have 1/8" markings you have to eyeball it half way between the 1/4" markings.
Sometimes patterns give those oddball measurements for squares to be sewn into half square triangles (aka HST, triangle squares or as in your patterns case she is calling them diagonal squares). What I prefer to do is make them oversized and trim down. So I would cut 3 1/2" squares and follow her directions to make the HST units. When they are completed you need to trim them down to 3" squares lining up the 45 degree line on your ruler with the center diagonal seam.
PS welcome to the board and pretty quilt.
Last edited by feline fanatic; 05-14-2016 at 11:15 AM.