Oh, you don't have to fill all three spots to trade. You can just choose the ones you're offering and hit the little blinking button near the bottom right-hand corner of the screen, next to the "Return" button.
Hey, sorry to keep you waiting. I'm trying to catch a specific Natured Ditto as well. I'm almost done. If I don't get it on the next try I'll go ahead and come trade with you regardless. I'll only be a few more minutes.