Jim Dixon, I'm glad MS Word's upgrades have corrected their SpellCheck so that it no longer "corrects" you if you type "sing-along," but they should also accept "singalong," as that is an acceptable spelling per most dictionaries, whereas "sing-a-long" is not.    I don't agree that all sequences of three words would be accepted as hyphenated words.   The point is, that while "sing," "a," and "long" are three valid words, putting them together as a hyphenated word does not fit with the meaning of the word "sing-along."   "Singalong" or "sing-along" means an activity in which people sing along with each other, not an activity where they sing a long (song or whatever).

Since there is no such dictionary word as "sing-a-long" (it's "sing-a•long," with the bullet indicating syllabication), any good Spell Checker should flag that as a non-word.