A 3 set is easier than a 2. You've got a higher probability of at least one clicking with you. Also, there's a higher comfort level on your part because they know you're at a disadvantage (i.e. outnumbered) if you're solo.
In this instance, judging strictly by the picture, the one on the right should be your target due to the slight separation in the 3 and, really, she's the most attractive of the set. Your back is to her (your right side target) slightly and you open on the center one. Include the far left girl for her input and gradually acknowledge your target as she attempts to include herself. Segue into all 3, but keep the target at arms length and qualifying to be in the conversation. Reward any of her IOIs with incremental attention. Stay C&F and Neg only the target if it seems appropriate and even then keep it light hearted. Disengage after 5-6 minutes and allow them to talk about you before returning (this is social proof generation). This time involve the target more, but stay entertaining. Isolate the target and enjoy.