Rubio can't string 3rd and 5th place victories in March and still have a path to the nomination, not with how few delegates he'd be earning. If Cruz falters and Trump dominates March it will be over because Trump will be racking up so many delegates that Rubio won't be able to stop him.
But I wouldn't expect the GOP race to narrow to 2 contestants for a long time (maybe never), neither Cruz nor Rubio will be dropping out lightly.
I mean, I think the most likely outcome is a Trump plurality and no majority. But Rubio could win Ohio and maybe 2-3 more states in March and still be in a decent position. Cruz basically needs to win the day on March 1 and March 5, hard, or he has no chance left.