Two Sets of Rules, the Good and Bad?

[MaxWilson]

Yes, this part of the rule is stupid. But it's kind of a corner case. In 99% of the fights played, there is always at least 3 enemies, so a legendary creature gets its 3 actions per turn, and is forced to spread them through the round, which is the intended effect.

In the 1% chance in which the legendary creature is against less than 3 enemies, the designer probably though:
(1) It might be a monster VS monster fight. We expect the GM to just use GM fiat to determine the result, not actually roll the attacks, so we're not gonna add a special exception for that.
(2) It might also be a monster VS very few PCs. The players probably have a rough time already. Moreover, we only added legendary actions to deal with the balance problem of "lot of heroes VS one monster", if there is not "lot of heroes", there is no need for the legendary actions any more. Let's not bother with adding an exception either.
(3) In both cases, if this bother the GM, that's why there have rule 0: to deal with corner cases and weird 1% situations which we willingly chose to not cover properly.

Legendary Actions are an awkward solution to a problem created by 5E's initiative system. They're not a balance fix at all, since legendary actions are factored into CR exactly the same as Multiattack actions.

Fortunately, as you say, a DM can simply rewrite the legendary action rules completely (100% of the time, not just 1%). I just have monsters take legendary actions at 5, 10, and 15 points slower on initiative than their regular action. This eliminates Schrodinger effects and removes the verisimilitude issue.