So mom gifted the home to the kids before it was sold, that would be the only way the 1099-S's were in their name. 

Say goodby to the $250,000 exclusion and hello to 1/3 of Mom's basis for each of the kids.

If she just told the title company "send this much of the $$ to each of my kids" the -S would still be in mom's name.

Just to be sure find out if the kids had to sign at the closing - that should be the real tell as to who the owner was at that point.

Somebody gave them some great estate planning advice there!

But on the other hand - you get to charge mom for preparing a gift tax return too!