What other languages have you "picked up"?
Surely at least one of those other languages used functions.
And surely those functions could take multiple arguments.
You do need to also learn to manipulate the browser Document Object Model ("DOM"), but until you can write a function taking multiple arguments, you aren't ready to do that.
If you have tumbled to what I have been saying, yet, then understand that you haven't even got the first line of your function correct. Read again the requirement:
Define a function maxOfThree() that takes three numbers as arguments