Well, kind of. That would then be a substantive adjective. You nominalize the adjective three-year-old.You could drop the word "child" and simply use "a three-year-old" as a noun.
Anyway, I no longer teach grammar, so I also no longer really give a ◊◊◊◊, but thanks to y'all for the suggestions.
Besides, it's also off topic, so let's return to the theme of the thread.