Are you wanting a single component to equal 3.5 inches? The way I make my 1/2 square triangles is I go up 1/2 inch bigger than what I want my finished square to be then trim.
3 1/2 inch unfinished 1/2 square triangle--start with 2 pieces of 4 inch square fabric, right sides together, mark corner to corner, stitch 1/4 inch on each side of mark, cut on mark, press open, trim to correct size.
4 1/2 unfinished--2 pieces of 5 inch square fabric
5 1/2 unfinished--2 pieces of 6 inch square fabric
If you want what the picture is to be 3 1/2 inches each component has to be 2 inches unfinished each.
When I say unfinished I mean before it is sewn together with any other parts.
Does this help?