An LMG is usually a Machine gun I.E. Mg34 that has an attached magazine or bucket for chain of bullets. It usually doesn't have a bipod, but if it does, it's really light.
An MMG is usually a Machine gun with a heavier set up bipod. It usually takes 2 people to fire it. One to carry the gun and some ammo. One to carry the bipod and some ammo. The person who isn't firing it usually helping to feed the ammo into the gun.
A HMG heavy machine gun has a reinforced heavy tripod. It's also fitted to sustain longer periods of being able to fire. It usually takes at least 3 people to carry it. One to carry the tripod, one to carry the gun and one to carry the ammo. Often, the tripod is heavier then the gun and is split imbetween two guys with everyone carrying ammo. It can also be set up on different supports.
Darn, world.guns.ru doesn't allow remote linking. But every gun can be modified to be fitted to different roles.
I.E. Mg34/42. The MG34 can have a light barrel with a 50 round magazine and a light bipod. It can also have a heavier barrel with a 150-250 round belt a medium bipod. It can also have a really heavy barrel, a really heavy tripod and 250 round belts.
Notice that it's roughly a .30 cal, yet it can be an LMG, MMG, and HMG depending on how you outfit it.