We all hate to trim, but there is nothing wrong with adding a full two inches to the finished measurement you want, and then trimming to the size you want.
Now, having said that, are you working to the finished measurement of the square you are working the triangle to - -?
or, are you measuring the square you are looking at, thinking that is the "finished measurement" ... not so! ... if you are looking at a square that is 3 1/2", it will finish at three inches.
Maybe we need a bit more information on the measurements you are working with and the sizes you want to end up with?
When in doubt, cut larger and trim down ... in time, the formula will work itself out.
Have you seen the quiltville site?
http://quiltville.com/
Bonnie Hunter has a lot of scrappy quilt patterns at that site. She helps with organizing scraps, AND! She has a lot of fun ways to learn how to make traditional blocks ... she really wants to help us succeed at making quilts and her snapshot by snapshot step explanation with well-written instructions should be able to help you understand what your mom is talking about.
Having said all of that, I am curious - - why aren't the measurements working for you?