The most common cause of rat RI's is
Mycoplasma pulmonis(commonly called "myco"), all rats have it but some are more prone to develop RI's from it...pneumonia often sets in as a secondary infection to myco...the best way to avoid having RI problems in your rat colony is to avoid buying rats that are constantly sneezing or that have red "snot" around their nose and/or eyes... here's a link to a very informative page about myco, hope it helps some...
click here!
Just thought I'd add my personal experience...two of my rats had mild-moderate myco infections their entire lives, I did have them on antibiotics for a period of about a month (baytril and doxycycline) but as soon as the course of drugs was over the symptoms came right back...since myco is chronic and the meds and vet bill for the 2 of them cost me over 100$ (and I was very strapped for cash at the time) I decided not to put them on another run of drugs...they both lived to ripe old ages (nearly 3 yrs, old for a rat) and when they died it was due to age related illness (tumours and degenerative myelopathy)
Not really similar to your situation, since I never had one die because of RI, but just mentioned it to let you know that the infection can always come back after antibiotic treatment, be it pneumonia or just mycoplasma...
Egads! Another thing I thought might be helpful: rat RI's can also be caused by a viral infection (which wouldn't respond to antibiotics) there are 2 that are "common": sendai virus and SDA, and there are others...so, you might want to ask your vet about the possibility of a viral infection...I say this because it seems odd to me that the 3 new rats would die right off like that unless they already had fairly advanced pneumonia (which usually has pretty noticeable symptoms)...also, the viral infections would be much more contagious (and more lethal) than the pneumonia and, therefore, pose quite a risk to your existing colony.
Some useful info on rat RI's for everyone:
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