What is the meaning of the three kinds of penetration?

In a Tank Characteristics, it says "Average Penetration 45/49/22 (in mm)". The same applies to damage, where it uses this split into three formats.

This is how much armor (in mm) it could penetrate, but why are there three numbers?

According to the chaps I found talking about it over here it's to do with the different ammunition types:

AP / HEAT / HE

penetration is related to the shell type. AP is the "standard" round HEAT has the most penetration, buts its bought with gold = you pay with real money HE has low penetration and high damage, but it will always do some damage when you don't penetrate the armor, its used against high tiers when your gun can't penetrate them.