We've had all three, and I'd say guinea pigs are the best, followed by gerbils. I'll never again have a hamster. My pros and cons:
Guinea Pig
Recurring costs: Food - est $3 a week. Bedding - est 75 cents a week.
Requirements - fairly large cage, like a BIG aquarium or a rabbit cage; large water bottle; hidey hole (I bought a big cheap bowl, cut a hole in it and turned it upside down); food dish (I got an automatic feeder, because a regular bowl had to be filled every two days)
Optional - exercise wheel or ball (if you don't have this, you need to let them run in the bathroom or something for exercise), hammock (DS's pig LOVES hers!)
Pros: Very social/interactive once they're familiar with you. My DS's GP (Misha) "talks" to me whenever I come into his room.
Big enough that little ones don't accidentally squeeze too hard.
Fairly clean (see con below).
Cons: Take up more room.
Drink LOTS of water, which leads to wet litter.
Can be noisy at night. DS complains about this sometimes, but it's usually when Misha's out of water. As long as her needs are met, she usually sleeps in her hammock or hidey hole.
Largest intake of food, so highest costs.
Gerbil: Recurring costs - food - 25 - 50 cents a week, bedding, 10 cents a week.
Requirements: Cage (small aquarium size, or they sell hamster/gerbil cages for about $20), small water bottle (comes with many of the cages), food dish (they can use just about anything).
Optional: Exercise ball or wheel, chew sticks (they like to chew things!)
Pros:
Fairly social (DD's also sits up and interacts, but she's not vocal). She also likes to be held.
Fairly clean (her bedding needs changing every other week)
Very easy care
Cons:
She's wiggly. Even though she likes to be held, she'll squirt out in a heartbeat! The GP will cuddle in your lap for a long time.
Gerbils are fairly rat like. My mom WON'T touch her, but she will pet the guinea pig.
Hamster: cost similar to gerbil but bedding is twice as much.
Requirements/options, similar to gerbil
Pros: Frankly, I didn't find any
Cons: All the cons of a gerbil, plus
STINKY! They make at least twice the mess of a gerbil. Cage really needed cleaning about every 2 days.
Much less social. We tried a hamster for DH (traded it back in for the GP), and it BIT really badly.
Hope this helps with your choice!