Single unit buffs are plentiful, battlefield ones aren't, am I correct? From that standpoint, I'd suggest scrapping the spell rather than making it a single unit buff.
As for what would happen if you have just one unit? Nothing more, it just guarantees that that unit is always lucky.(I suppose meaning you can use it as a single unit buff(or 3 unit army), that are always lucky. Essentially meaning the spell is actually the best when used with an army of 3 powerful units rather than a big army(as it was originally intended)). Thus, it really does need to be changed after all. I think kilobug's version seems more reasonable as it prevents "abuse" in one sense.(but what about units with multiple attacks?)