Sexing is very easy once they get to a certain size (about 5-6 cm) The males will have big bristles coming off his nose Whereas the female will only have very small bristles if any at all. (maybe do a google image search on female and male bristlenose....??)
As for encouraging to breed, I would probably get 1 male and 2 females, feed them up lots and lots on different foods like cucumber, lettuce, frozen bloodworms, pleco pellets and anything else realy....
Bristlenose can be a bit wierd when breeding. A male and a female may breed non-stop for 12 months, laying every month, then they will suddenly stop, and everything you do, they will not breed again for another x amount of months, Then they will suddenly start breeding again.....
Good luck.
EDIT: I posted the same time as fishboi