All people are different, and all relationships are different. L could be looking to fulfill a fantasy as some kind of fix, but you can't really know that for sure unless you ask him outright (and even then he might have to do some introspection to know).
It sounds like you and L generally communicate quite a bit, which is a big advantage toward your success in this FFM triad prospect. I would just take things really slowly, and check up with each other often along the way. Other than that, your prospects are good, even though true love always comes with some risk.
This is a good site, with much info and many resources. Take your time, to look around, and see what threads call to you. Continue to post any thoughts, questions, or concerns you may have. It's quite a busy site, so ping your threads from time to time as needed.
I am personally in an MFM triad, so my advice for you is necessarily generalized. But like I said, take it slow, continue to communicate, and learn as much as you can about how poly works. Polyamory.com is a good place for learning that stuff.
Love means never having to say, "Put down that meat cleaver!"