I've read that you need to pass the Series 3 exam in order to trade currency for others, specifically here in the USA.

Is there any truth to this statement? Or is it just "recommended" that you have a Series 3?

Used to be that you could trade for anyone with just a POA. But with all the new regs, and more coming, I don't know of a single US FCM that will allow you to use their MAM unless you are registered. Technically the rule of under 15 still applies, but they want you to be registered. If you're just going to trade for a couple freinds or family you'll probably be okay, but you won't get the MAM.

Not only passing the series 3, but you'll have to join NFA as well.

This only applies to USA, by the way.