Have you ever tried to put a Large creature mini in the center of 3 1″ hexes like the 5e DMG indicates is the size of a Large creature?

That mini’s base is spilling waaay over into the bordering hexes, such that no Medium base can fit there. And a Huge base swallows the 7 hexes the DMG says it’s supposed to get plus takes a bite out of 5 more hexes.
Either you need to use a much bigger hex grid (like 1.25″, maybe 1.33″?), or just drop the 3-hex configuration and bump the size categories up so that a Large mini fits over 7 hexes, Huge covers 12, etc.
And this makes way more sense because a 1″ hex isn’t a square inch of area the way a 1″ square is. A 1″ hex is ~.853 square inches, so 3 of them together is only about 2.56 square inches, significantly smaller than the 4 square inches on a square grid. Currently, the DMG recommends an increasingly smaller area on hex grids than on square grids:
- Medium (1″ diameter base, 0.785 sq. in.) = 1 square (1 sq. in.) = 1 hex (0.853 sq. in.)
- Large (2″ diameter base, 3.141 sq. in.) = 4 squares (4 sq. in.) = 3 hexes (2.56 sq. in.)
- Huge (3″ diameter base, 7.068 sq. in.) = 9 squares (9 sq. in.) = 7 hexes (5.974 sq. in.)
- Gargantuan (4″ diameter base, 12.566 sq. in.) = 16 squares (16 sq. in.) = 12 hexes (10.24 sq. in.)
Notice that every hex arrangement over Medium is actually smaller than the creature’s base? You see how the Gargantuan creature is supposed to take up barely over 1 sq. in. more on a hex grid than the Huge creature does on a square grid? These hex arrangements are unworkable.
The Huge creature should take up 12 hexes, the Large 7, and the Medium 1. Gargantuan, then, takes the 19-hex pattern that Colossal creatures took in previous editions. (The 19-hex pattern covers 16.214 sq. in., almost exactly the 16 that Gargantuan creatures on a square grid takes).

If you want, you can put the Large creature on a symmetrical 6-hex pattern that actually makes its total area closer to the 4 sq. in. of the square grid than either 3 or 7 hexes does (5.12 sq. in. instead of 2.56 or 5.974, respectively).

I know no one cares about hex grids anyway, and VTTs and TotM really don’t care how big minis are. But it bothers me that multiple editions have been suggesting such absurd hex conversions to ostensibly use with your same minis and maps.
/rant