Even if Santorum and Huckabee don’t endorse Cruz, when they drop out, most of their supporters will probably go to Cruz because of his strength with an Evangelical base, though some of them no doubt will go with an establishment candidate like Rubio. I think much of Rand Paul’s 2-3% will go to Cruz as well, since Cruz has effectively co-opted many of Paul’s libertarian positions (NSA surveillance, etc).
Time will tell.