You can calculate 3???? = 26796, which of course is greater than 1000.
The joke is that they aren’t asking you to calculate 3????. They are asking you to calculate 3???. (This is 231.) The last question mark in the sentence “What is 3????” is the punctuation that ends the sentence, not the postfix notation for adding the first n positive integers. You could rewrite the sentence as “Find 3???.” Of course, that wouldn’t be as funny.