They were a week old when we got them. We acquired an older hen from a different flock back in December, she was put in quarantine for a month before joining the other two.
So they grew up with anything that might be there. I highly doubt they are carriers of anything, then, as I believe you said they have never shown any symptoms of anything. As for the little ones getting sick and dying, there are so many variables that could cause it, it's really hard to pick one - even a wrong heat lamp bulb for their heat can sicken and kill little ones.
Nor can you entirely blame the source for the ill chicks, either, as they may have been just fine when they left their birthplace. And just by traveling with them to your home, they could be exposed to something. If someone with a sick flock goes to the same store you went to, and didn't practice good biosecurity, like wearing the same shoes they tend their chickens in, you could bring back something unpleasant or even dangerous.
Really, the only thing I would do (as I don't treat illness with antibiotics, just wounds) is give them a vitamin/electrolyte booster like Durvet sells (water soluble, they have a measuring cup with the bottle, 1 cup per gallon), and add ACV (with mother) and honey as well, 1 filling/day the vitamins, the next filling/day the ACV & honey, and add probitics to their water with the vitamins (not with the vinegar - and Durvet sells the probiotics, too), all to boost their immune systems. Then see who lives and who doesn't. *shrug*
A little heartless? Maybe. But I don't breed those with weak immune systems. I breed for immunity. And if you decide to get more little ones, try to stop at as few places as possible while traveling with them. And I always give my little ones that vitamins one day/ACV & honey the next, whether I hatch them myself or whether I buy them elsewhere, for the first few weeks. I always watch to make sure they are drinking it, though, as some seem to not like the treated water, and if that's happening, I go at least one day without anything in the water in between one day of the first mix, then clear water, then the next mix.
Anyway, that's my 2 cents worth.