I don't really believe any of the three are hiding anything on their tax returns.
What you or I believe is immaterial. The fact is, we don’t know.
When you don’t release the entire document, the implication is that something must not be revealed.
Since Romney put it out there, that there must be a “Bombshell” lurking in Trump’s tax return, the implication is justified in the case of Rubio’s lack of completeness, and Ted’s lateness.
More than one can play this game.