I'm in the process of making an I-Spy quilt that uses hexagons and triangles. My hexagons are smaller than yours, though, but maybe my measurements can still help you.
The triangles are equal length on
all three sides and the length of one side of the triangle is 1/2" larger than one of the sides of the hexagon. So, for your hexagons with 3-1/2" sides try a 4" triangle (all three sides are 4") and see how that works. I cut all of my triangles out of strips....since all sides are equal I just keep rotating the ruler and can cut 4 layers at a time. Is that clear as mud?
When sewing the triangle to the hexagon it will be just a little bit wider than the hexagon but that is alright because it will even out when you sew the 1/4" seam. I can post pictures of how it's done if you still need help. It's not really as confusing as it sounds.
Here is a link that shows you what I'm talking about
:
http://www.americaquiltscreatively.c...es/AQc-612.pdf
This one is even better:
http://quilt-knit-run-sew.blogspot.c...-tips-and.html
Even if yours isn't an I Spy the process is still the same. Good luck with your quilt.