P90X has you use a chair to assist. Put the chair in front of your pullup bar, plane ONE foot on the chair (the other leg hangs straight down to catch you if you fall off). No use the leg on the chair to give you as little assistance as possible, but still do the pullup. I will usually max out on unassisted pullups, then add on about 10 assisted pullups to keep getting stronger.
BTW, I think 3 unassisted pullups is great! Keep building,