Generally, it shows confidence just to maintain eye contact for a few seconds. Just don't look away immediately, this shows insecurity.
In your scenario with the three girls, I would make eye contact with the 1 I find most attractive. When our eyes meet, I would smile. If she smiles back, I would say hi. She'll probably say hi too.
If you notice anything interesting about the interaction (they stop, she has a seductive tone, her friends start giggling, etc.) roll with it and see where it goes. If you do everything with confidence and charisma, you will at the very least get a positive interaction out of it.