Surely you realize that it depends on which three months you are talking about? Jan-Mar is only 90 days (unless leap year), Jun-August is 92 days.
In fact, by my calculations, there are only 2 contiguous 3-month periods in the year that add up to 91 days… April thru June and September thru November (there are two additional periods if February falls in a leap year… January thru March and December thru February).
But 3 months adds up to 92 six times (fully half of the time) regardless of leap year.
If you try to vote the day before your 18th birthday, or try to get served alchol the day before your 21st birthday, don’t expect anyone to sway anyone with “…no really, you forgot about leap year”.
As far as being LEGALLY 21 as inquired in your OP, your are LEGALLY 21 when the powers that be say you are… regardless of how many days, hours, or minutes have passed since you graced the world with your presence. Don’t hold your breath waiting for them to change no matter how much sense it makes to you.