How can we keep a hamster alive?

Question
We got a hamster for my 9 year old daughter for Christmas.  We did lots of research and chose a golden hamster from a good pet store.  She lived 5 weeks and died in her sleep.  She did not appear to have any health problems..was eating and playing normally...one morning we found her curled up in her house, dead.  Hamster # 2 (also golden) had wet tail and died within 3 days.  Hamster #3 (teddy bear) was just like #1...seeminly healthy--eating normally and active. She lived one week. We found her today curled up in her house, dead. Both seemed content and not at all stressed. My daughter was very careful when handling them.  Needless to say this has been devastating for her.  She is about ready to give up on hamsters.  Can you help us?  Are we doing something wrong?  We are giving the food recommended by the pet store, putting vitamins in the water, keeping the cage clean (we disenfected it between hamsters), etc.  What could have happened to #1 and #3? What can we do to keep this from happening again?

Answer
Wow, thats an awful streak. The only hamster I had die on me untimely was within 14 days, he was very mean, so I'm guessing he was sick. What type of bedding are you using? If you have any of that cotton fluff stuff in the cage, get it out. That is actually very bad for hamsters, and I don't know why pet stores bother selling them. The type of cage may play a factor as well. As for vitamins, don't bother, I've only tried the vitamins in the water with one of my hams, and he didn't live any longer or happier then the others. It ended up yellowing the water bottle over time. Vitamins are only really necissary with very small hamsters, (up to 12 weeks) pregnant hamsters, and old hamsters.(1.5 years +) Try a different store, my guess is that pet store is not getting them from a reliable source. Even though a pet store may seem good, I know what goes on behind the scenes. They may all have been kept in the same infected cage, or come in contact with a sick hamster, in effect killing off the whole bunch. Try a different store, sometimes they just end up selling hamsters that are ill and can't deal with stress, even the shock of moving. Over all I just don't have enough information on what might be causing this.