neo3 is neomycin and triple sulfa combined in one capsule. in fairness, while it's easy to use - you can purchase both drugs separately and dose individually. liquid neomycin can also be found as "biosol" at feed stores. there are a number of other antibiotics that can be used as well.
what symptoms does your horse(s) have? wild caught? captive bred? pen-raised? let us know and we may be able to help.
i am currently using Furan 2 but won't muddy the waters here, so to speak, on why i am using it. as a first choice antibiotic i would use something like neomycin/triple sulfa, gentamycin, etc. but we need to know a little bit more about the symptoms your horse is displaying. i'm not as quick to reach first for antibiotics at the first sign of trouble, there are a number of diseases with symptoms similar to bacterial infections.