If you've played C3/C3 HD on original difficulty, you have no doubt encountered the dreaded medikit bug:
Watch as a Commando get shot by an MP40 once and somehow the 5 mediktis he was carrying vanish into thin air, as seemingly all 5 medikits were used once.
However 3H on the new Rookie difficulty fixes this, presumably unintentionally or they'd have fixed it for original.
My theory is that once they realized C3 was either A: too short and/or B: how kinda overpowered guns were in C2 (After all, they nerfed ranges hard) that they nerfed medikits as either or both a way to discourage combat and make the game longer.
BUT the game's "auto-heal" system is (my theory, I haven't checked the code) seemingly checking just if "Commando health is lower than Medikit's maximum healing".
So with the low healing where several doses are needed to heal above this threshold this seemingly causes the game to instantly use several medikits instantly. (which HD Rookie fixes by having Medikits healing almost all your health.)
That said, I have discovered a workaround that works in Both C3/C3HD for allowing you to always make maximum use out of medikits:
Find either a box or enemy corpse and store your medikits inside it and then after you take damage, take a medikit out individually (By holding shift) and then close your inventory after sustaining damage.
If you've sustained enough damage, it'll be auto-used, now repeat this process with the remaining medikits until they're no longer auto-used then put any remaining medikits back in the box.
You can test this yourself in "Stop the Train", kill the MP40 Gestapo near you at the start with your pistol to lure him in, place the medikits on his body and then have the nearby worker punch the Green Beret to near-death then kill him, if you take the entire stack off the body, they'll all be auto used for minimal health but if you take them out one at a time closing the inventory after each medikit, it only actually takes 3 medikits to nearly fully restore the Green Beret's health.