There seem to be only two options.
Scan > Settings > Let avast generate a report file. Then fill in the filename prepended by its path; either X:\share\ or in UNC form \\server name\share\
If neither works, then there is no direct way.
EDIT You likely cannot output from 3 pc's to one log file, due to filelock problems. If you could, I'd try to choose a different interface language for each pc.