E.M.
I had this same problem with my daughter when she was 3. She would always hide for #2 until she was done. Finally, one day I very matter-of-factly informed her that she could poop wherever she wanted -- her choice -- but it would be her job to clean it up. So I very earnestly and sweetly showed her how we dump it in the toilet, then we rinse it in the sink or tub, and finally, take it to the laundry room. I may have (gently) had her help me hold the panties while I was showing her. I did not display any negative emotion, or act frustrated at all. She was fully potty trained from that moment on.