The title says it all: if you'd like more Fire-Emblem-playing 3DS friends, respond with your 3DS friend code.
If you have yet to even use your friend code, you can find it by clicking the little orange smiling box at the top of your home screen. Your 12 digit code will be listed on your Mii's card, under the name. This is also where you will add friends. I'm pretty sure that no notification is sent to an added friend: they must manually add you to their list as well.