Piece 3 is the back (cut it on the fold). Piece 2 is the front piece. Cut 1 back piece on the fold and 2 front pieces from each of your 2 fabrics. To make your own pattern, I would suggest tracing out the front and back pieces of an existing pinafore pattern. Use the back as is and then adjust the front by drawing a curved line from one shoulder down to the armhole on the opposite side. I haven't tried making a pattern this way myself so I'd recommend trying it out on some cheap fabric first.
To make the dress up you do the following:
1. Sew the side seams of both dresses. The shoulder piece of each front piece laps over the chest to the opposite shoulder.