After some experimentation, I hit upon an envelope for 3 x 3 cards that I like..they are not perfectly square, which is nice in case your card has bling or dimension.
First, you'll need an 8" x 8" piece of paper (which will make 2 envelopes. Cut in half so you have 2-4"x8" pieces of paper. Put one aside.
Score 3/8" along each long edge as shown in photo below:
Turn the paper the the other way and score at 2" and 5.25". Cut out the pink areas as shown in the diagram below. You can use a corner rounder on the top and bottom flaps. Fold in the side flaps. Apply glue as show in diagram and fold up the bottom flap. Voila! Really cute envelope!