You might be able to find samples for a double bass drum. You could probably just use 2 bass drum samples and put them close together... Not sure lol never done that before in FL
I don't use FL, but isn't there a possibility to speed up 1 beat 2 times? That's basicly the same as a double bass drum if you just want to keep it going.
Unless you have three feet, triple bass is impossible lol. Double bass is where you have two kick pedals and you alternate very quickly one from another making a drum roll but with a kick. Used mainly in harder styles of rock.