A.R.
Drop the "bad mom" stuff because it will snowball on you M.. How can anyone be bad who is raising twins and a son with down syndrome? Take a step back for a moment and:
Read to them: Once Upon a Potty and Everyone Poops. They need some encouragement from someone other than you. Also, seek out stories that include children who use the bathroom (as a librarian for titles).
It's good for them to know other people who use the bathroom since their brother doesn't. Of course, they will think he is normal so they should follow.
By the way, do YOU take them to the bathroom with you so that they can see how you use the toilet, wipe, etc? This would be a good first start.