In amateur radio terms, the numbers 73 are an abbreviation for “best wishes” or “best regards”. Note the plural.
At the end of a contact people generally sign off using that abbreviation, it’s polite.
But if you say “seven threes” or “seventy threes” then what you’re really saying is “best wisheses“. That sounds a bit silly.
If you want to make it really bad then you could say “best seventy threes” at the end of a QSO which translates to “best best wisheses” which is even worse.
It’s 73. “Seven three”. At a push it would be “seventy three”. It’s already plural, no need to add an ‘s’ on the end.