Well, for EUR or GBP, --limit does the job, for example:

ledger -f main.ledger bal -l "amount =~ /GBP/"

I presume that would work for "USD" too. If you have "$" as your currency symbol, you'll need to work out how to quote that correctly for the regexp and the shell.

Also, it is possible to use this:

ledger bal 'expr(commodity=="USD")'

It's really a matter of taste. Some people use one big file. Some organize by account, some by year, some by month. Choose what feels right to get started, you can always change it later!