Online gaming on a PC will use maximum 100MB /hour - usually around 70-80MB /hour.
This is regardless of the type of game, because graphics do not factor into data consumption, meaning games like Minecraft and Call of Duty will use the same amount of data.
However, one important aspect to consider is ping time – mobile data connectivity can be a lot slower than your home LAN network. For this reason, players may experience lag while playing simply because the internet speed is insufficient (e.g. when connected to 3G). For games where success depends on reacting quickly (shooters), a 3G connection may be too slow for the game to be playable.