If you want to make 3" finished half-square triangles, your strips should be cut 3-7/8". I personally cut them 4" and trim them down a little. You don't need any special rulers. Any ruler with a 45 degree angle should work fine.
When you have your strips cut, mark them into squares the size you cut your strip. Then mark corner to corner on the diagonal and sew 1/4" on each side of the drawn line. Cut them apart on the drawn lines. I cut the lines for the squares first, then the diagonal line. Start in one corner of the first square (say the upper left, down to the lower right), the the opposite direction on the next square, across your strip. This makes it easy to sew continuously across the strip. Then turn around and come back across the strip on the opposite side of your diagonal lines. Each square will make 2 half-square triangles. Press them open, then square them up. Your easy angle 6 1/2 ruler should be perfect for this. It's the one I use.
Hope this helps and doesn't confuse you more.