You could try setting up "symbolic links" from one folder to another. On the command line you'd use
ln -s /path/to/qa-plugin
So you'd put the plugins in /site1/qa-plugin/ and then go to site2's directory and run that command using the path for site1. I don't know 100% if it will work, if certain plugins write to files or options specific to a site it could cause errors.
Alternatively, if you have access to the command line you could write a shell script that copies everything from one folder to the others, e.g.
cp -R /path/to/site1/qa-plugin/ /path/to/site2/qa-plugin/;
cp -R /path/to/site1/qa-plugin/ /path/to/site3/qa-plugin/;