I was part of a triangle like that. The two of us that hung out together most often, lived closest to one another; it was more convenient for us to get together. If you and B are neighborhood/past friends and L works with B or something, then B does have the best of both worlds and it could be that right now whatever real-world connection there is with L might be "easier"/fresher/newer than what she has with you. I might look for other friends of my own too so I don't feel so vulnerable to not having anyone to do something with.

Too, I think I would try to come up with a couple activities and invite one or both to them, rather than wait for them to invite you. If you "feel" like a third wheel, you might be acting like one? We have to ask others to do things as much as we wish they would ask us to. It sounds like you are closer to B than L too; I would try to do some things with L, alone, if you like her and would like her to be a good friend. That might help make the threesome feel stronger to you also.