Trump versus Sanders and Billary, whether in a race for the Democratic nomination or the general election, wouldn’t work.
Right-leaning democrats would go for Clinton, moderate conservatives would be totally disenfranchised unless they broke ranks and voted for her as well. People who are inclined to vote Sanders would never support Trump anyways.
Basically, the only votes Trump would get are from those who fear Muslims and Mexicans more than they fear being excommunicated from the Republican cult. That would not be very many either as they care so much about conformity that they don’t even bother forming their own opinions; they merely repeat something they heard from some frothing-at-the-crotch lunatic with a blog.