score:4
Frankfurt Airport offers luggage storage in both terminals, at 7 EUR per 24 hours for normal bags. It sounds like it's a staffed service, not lockers.
As for the train station lockers mentioned by Gagravarr, be careful because they actually have a maximum storage time of 72 hours, so if your "about 3 days" is a little longer, you might have some hassle getting your luggage back from the lost&found. Additionally, you need to have the correct coins for the fee.
Upvote:1
As with most things rail related in Europe, your best bet is often Seat61. In this case, the Seat 61 page on Left Luggage in Germany:
most other main stations in Germany have small & medium lockers, open 24 hours a day, for €3 & €5 per 24 hours
For your specific case of Frankfurt Main, you need the Bahnhof page for the station which shows that they have left luggage lockers (Schließfächer) there
So, 3 days will set you back either €9 or €15, depending on the size of locker you need