Would note that 3M is an unconventional was of representing 3mm or 3mn.

I’ve never seen ‘mm’ nor ‘mn’ used in British English, where ‘m’ is a common abbreviation for “million”. But this might be a localisation.

Wikipedia does recognise ‘m’ as an abbreviation: https://en.m.wikipedia.org/wiki/1,000,000

It’s definitely open to confusion but generally if you already known that abbreviation exists then one can usually deduce how to interpret that abbreviation from the context of the sentence. In this case, it’s a financial headline so I assumed it was “million”.

Millimeters or mili-newtons? Wait...that'd be mN. <confused> Obviously M is the conventional way to denote 10^6.

I got downvoted due to someone's unconventional reading. You win some, you lose some.