[Yizahi]

Gamers just write it as 3kk. (PS: never seen MM used as a "million" even in my two decades on the internet)

[kukkamario]

Eh... Haven't seen anyone using 3kk to abbreviate 3M. 3M is common and B for billion. Those are also used in many games to shorten currencies.

[squigz]

This seems to be a (IRL) cultural thing - the vast majority of people I play EVE with use k/m/b/t, but a small percent does use k/kk/kkk

[Yizahi]

Afaik it came from the first Korean MMOs, like Lineage. Maybe even earlier, but I saw such format there first.