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The acrylic rulers usually have markings on them for the finished size of the piece you will sew and the size of the piece you will cut for it. Here's an example of a 60 degree ruler where you can see the sewing lines and the cutting lines. If you want a triangle 6" high, from base to top point, then you would cut the fabric at the 6 1/2" line (which gives you a 1/4" seam allowance all around). For your 3" triangle, you'd use the 3 1/2" cutting line. Usually, people will cut a strip of fabric using a straight ruler, for example a 6 1/2" strip, then use the triangle ruler to cut the two other sides of the triangle.

Acrylic rulers tend to be more accurate for cutting fabric than if you use a paper or cardboard template to which you have added a seam around. After tracing or cutting around such a template many times it tends to gradually get smaller.

If you do what your pattern says, then you should be cutting and sewing the correct size for it. You certainly can compare the template you have made to the ruler and choose the correct line to use on the ruler.