Full body on each session usually works well for 3 days per week. That way each body part gets a stimulus several times a week. Pick a couple exercises each from lower body, upper body pull and upper body push. I'm also a fan of the flexibility of this approach since if you miss a day due to a busy schedule it doesn't matter too much.
Unless you're an experienced lifter, probably better off using an established program rather than coming up with your own. But if you do make your own, rotate the exercises for each lifting day. In other words, have 3 different routines (one for each day) rather than do the same workout 3 times per week.