Nobody's ever tried to claim the President lacks authority to pardon for crimes untried. What would happen if it was tried is unclear; there's no precedent. Trump's enough of an outlier that his successor might actually test the hypothesis.
The Constitution makes no mention of trial, or of conviction.
Indeed, the Supreme Court has ruled that pardons can be offered prior to a conviction in Ex parte Garland[0]
However, subsequent rulings call into question the absolute scope of Ex parte Garland (https://digitalcommons.law.wne.edu/cgi/viewcontent.cgi?artic...).
In particular, in case of dispute, the Court has the authority to adjudicate the dispute. It's the authority the Court used to hear Ex parte Garland in the first place.
And to my knowledge, there has never been a case that considers the President's ability to pardon for all possible crimes that could be filed related to some action. So if a subsequent President chooses to tack on a fourth crime to the list of three Snowden is currently charged with, a Trump pardon would likely not attach. At least, it would be up to the Court to decide if it attaches.