Right now there are 3 Republicans vs. 1 Democrat (your premise). Once it is a 1 on 1 it s over for Trump.
Example go look at the latest poll from Maryland (RCP)
Trump 34, Cruz 25, Kasich 18, Rubio 14
74% of Rubio and Kasich supporters got to Cruz 14% to Trump according to the latest ABC poll.
That would make the Maryland poll (not exactly a Cruz stronghold) Cruz 49 Trump 38.
Game Over.
Well that is a good hypothetical look at what some poll says but actual results show that as more and more candidates drop out, Trump continues to win. He is getting a portion of the support from each of the candidates that drop out.
An inconvenient fact . . . and from one of the most leftist big government worshiping states in the U.S.A.