First of all, for the position of the currency code itself, I found this interesting piece on Wikipedia:
The ISO standard does not regulate either the spacing, prefixing or suffixing in usage of currency codes. According however to the European Union's Publication Office, in English, Irish, Latvian and Maltese texts, the ISO 4217 code is to be followed by a fixed space and the amount:
a sum of EUR 30
In Bulgarian, Croatian, Czech, Danish, Dutch, Estonian, Finnish, French, German, Greek, Hungarian, Italian, Lithuanian, Polish, Portuguese, Romanian, Slovak, Slovene, Spanish and Swedish the order is reversed; the amount is followed by a fixed space and the ISO 4217 code:
une somme de 30 EUR
As for the prefix, M is the correct one, I guess it is because in metric (SI) prefix system M (Mega) denotes 10^6 (6th power of 10), and m (milli) - 10^-3. And there should be a space between the number and the prefix.
Finally, if you are not using these statistics in literature, you can use simple notation:
- < JPY 3 M
- JPY 3 M - JPY 5 M
- JPY 5 M - JPY 8 M
- > JPY 8 M
Though with the currency symbol (¥) instead of ISO code, as @WS2 suggested, it looks better.