Some folks shy away from tackling Y (or inset) seams but in truth they are no more difficult than sewing a mitred corner. In the picture below you can see the Y shape formed at the intersection of the three seams.
Whether your angle is 45°, 60° or even 120°, the method for stitching the Y seam is the same.
You sew the first piece to the correct side up to the marked X.
At the X, lock your stitches and clip your threads.
The seam allowance at the end is left free.
Voila! A perfect Y seam.