[souffle]

I'm so sorry to hear this. I doubt it was anything to do with you or your care but more that there may have been a problem in the store. Wet tail can spread from cages and bedding used between animals and handlers. The older hamster you adopted could well have been from the same batch as the baby you bought 3 weeks ago if they said she was 2 months. Sometimes groups or lines have something genetically wrong or are carrying an infection which can lead to them sadly passing young.
I would suggest doing a really good clean. Throwing out all the items that you have that are wood or cannot be cleaned. Replace the water bottle (the store should do this as this is twice you have got a hamster there that died) I would then try and find another source of a baby hamster be it another pet shop / garden centre or a breeder.
Tristar hamsters are in Bath and they have a website with a contact detail on it. Maybe leave the cage for a few weeks so nothing can stay in there.
I hope things work out better soon