Zero is not plural. The plural of zero is “zeroes” as in “The score for the team today is zeroes all across the board.” (Baseball)
But when zero is used as an adjective to modify something countable the word it modifies is expressed in the plural. Examples from other answers are “zero books”, “zero liters”, and your own example, “zero 3s”.
The reason for this is that zero is not one. In English usage, all numbers but one, when used as modifiers, throw the noun that is modified into the plural. We have no way to express a zero amount of a thing, only a singular way and a plural way, and since it is not singular, it is plural.
This is true also in other words that express negation. We say, “There are no ducks on the pond” and “None of the light bulbs are working.” There is no noun form to represent a zero number, only singular and plural.
