[Hammy992]

Hamsters are solitary animals and always do best on their own. In the wild they would rarely encounter other species and if they did they would keep their distance. Syrian hamsters must never be introduced to another hamster (or other animals) as it will cause stress and aggression which can result in the animals killing or seriously wounding each other.
Dwarf hamsters can occasionally live with a sibling from the same litter but they almost always fall out with each other and need to be separated. Like syrians, they cannot be introduced to other species.
Hamsters very much enjoy the companionship from their owners and do not need animal playmates. So your gerbils and hamster should be handled and played with individually. You should wash your hands well with a low fragrance or unscented soap before playing with each as the scent of another animal on you will be very stressful to them.