for the HST - half square triangle units - made of two triangles - the general formula is to cut the squares 7/8 inches larger than the finished size of the unit.
If you want a 3 inch "finished size" unit made of HSTs - then you cut your square 3-7/8 inches.
Many people have learned to add ONE WHOLE INCH to the finished size - for a "finished size" 3 inch unit -to cut the squares four inches and then trim down. Most of us trim off about 1/32 of an inch.
You can try what you want to do and see what happens. Some of us have to TRY whatever to believe/trust the advice we have been given.