You know the pull-up is a great exercise, but right now you may be too fat or just too damn big overall to get more than 2-3 reps. Keep working at it. But if you want to build your lats, you'll need additional exercises that allow you to get more reps. Try these:
The Rack Chin
These are chin-ups or pull-ups with your feet propped up on a bench in front of you. The Smith machine is a good place to set this up.
These are great for building wide lats even if you are good at regular pull-ups. I still do chins about every other week, but if I'm actually focusing on building lat width this is my go-to exercise.
You can get the lats a bit more lengthened than with regular chins because you can get into spinal flexion due to your feet being propped up on the bench. You can actually round the spine with rack chins at the bottom to really stretch the lats. You'll get a much stronger contraction at the top because you can arch very hard and really shorten the lats.
You want your legs to be parallel to the floor when you're at the top of the movement. A downward angle of the top of the legs is okay too. Just make sure you don't have a leverage by having the legs too low in relation to your torso. Add chains when needed. It looks cool.
The Assisted Pull-Up Machine
Don't shy away from the Gravitron-style machine. Do these after your standard pull-ups to crank up the reps and the volume.