I have 3 unit bridge 1 tooth infected. Any chance extracted bad tooth? cut bridge off from 3 unit bridge to safe other 1 tooth?

Sometimes that works but it's not usually a long tem solution.  The solution with the best prognosis is to place an implant in the space where there's no tooth.

Yes, the infected tooth and the false tooth can be separated oftentimes from the other crowned/bridged tooth so that it can be saved. If the bridge is all porcelain ,there may be more of an issue with breakage when it is separated. It would be unwise,depending on the location of the bridge, to leave the false tooth there as it would be hanging out in space and the chewing forces could harm the tooth left supporting it.I definitely would look into having an implant placed in the fake tooth spot and/or the extracted tooth position or both.