Okay at one point you said 3", at another point you said 3.5". If you cut your strips at 3" you will end up with a 3.5" border. This sample I made measures 3.5" at the top and bottom corners of the green fabric, and the whole section including the white fabric measures 4".
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If you want your finished border to measure 3", you will want to cut your strips at 2 1/8".