The theme is found at 53-Across: Average… or a literal hint to 17-, 26-, and 42-Across. Answer: Par for the course. Common expressions are turned into slang golf phrases by changing one of the words. Those substituted words are literally the number in golf scores that are under or over par.
17A. Neil Armstrong declaration = The (Eagle) two has landed.
26A. What a parent might warn a child to watch out for = The (bogie) one man
42A. 1961 Tony winner for Best Musical = Bye Bye (Birdie) one
I played golf many years ago (admittedly so bad at it despite taking lessons that I gave it up pretty quickly) but never heard of these slang expressions. So, it took me a while to figure out what was going on here. Overall, this was a fairly easy Thursday puzzles. Nice to start the year on a high note!