D'oh!

I don't know how I didn't think of that! If the first hash keys don't exist in hash2 or hash3, then they key doesn't exist in all three, does it. Likewise, if hash2 or hash3 have keys hash1 doesn't, same. ;) Sheesh, leave me alone and go home Monday...

OP... This answer by Eily is by far a better, more efficient and more elegant approach than what I presented.