Bother. I made mistakes. The first one was that I didn’t double check, and the three panels are different lengths. I thought I could deal with that after doing the side seams, but the sewing machine feed dogs messed things up further.
I’m going to unpick the seams, cut the panels all to the same length before seaming again, and then either sew them together using my walking foot or by hand-basting and then seaming. Haven’t decided which feels more appealing to me yet, but there’s time.