[Pebbles82]

Hope the carbon monoxide gets ruled out. It could be viral if the dwarf's cage was used for one of the Syrian's. Or it could be a combination of factors such as genetic predisposition combined with a lack of a nutrient in the diet. The Kaytee mix is not good quality I believe. Not enough to kill a hamster on its own I expect but could lead to nutritional deficiencies. There isn't much choice in the US I believe for a good hamster diet. Using Harry or Hazel hamster is a safe bet as they are sugar free, have enough protein and are tried and tested. It's recommended by the National Hamster council in the Uk. They are identical formulas but labelled differently due to local regulations in various countries - Hazel is the US version of Harry but I think you can get both.

I guess you just need to rule things out one at a time before getting another hammy. eg the detector, a good brand of food, making sure any cages are disinfected and then well aired (the smell of some strong cleaning things can make them ill). What bedding were you using? Bedding can get damp or mouldy if not stored inside. Most beddings are fine unless a hamster has an allergy to a particular bedding (usually a wood allergy) but it's unlikely all three hammies had the same allergy and you'd have noticed a lot of itching and sore patches and wheezing.

There was a thing with Carefresh a year or two ago when they started putting baking soda in it and people complained it caused illness and even death in hamsters. They don't do that any more as a result, but there can still be some old batches around, so if it's Carefresh check the label - it should say if it has baking soda in or not.

A good safe bedding in the US is Boxo (shredded soft cardboard) which can be bought in bulk. Or Kaytee Clean n Cozy. A lot of people like Carefresh as well - Boxo works out cheaper! Or you can mix a couple of them to make a more expensive one go further.