Question about cardio in new weight lifting workout routine.?

Hey guys, i recent;y started a new workout routine to get a little more fit. I'm not overweight, my BMI is about 15%, which i was told is in the normal range, but to be a bit more lean, i should stride for 8-9%. Now, at the same time i want to build some muscle, so my routine consists of a three day split, Monday, Wednesday and Fridays. Now my question is how should i fit cardio in this week? Only on my days off, two days a week, or should i go for the weekend too? Yesterday (Friday) i did legs, now my legs are pretty sore right now, but i could go for at least a jog now if i wanted to. Should i even bother, or should i just let them rest and recover the weekend? And by cardio i don't mean anything too stressful, about a half hour, 30 secs run, walk for 1 min and so on. Thanks, guys, any help would be appreciated, still trying to figure this whole thing out.