Hello,
W32.Changeup.C is a worm that spreads through removable and shared drives by exploiting the Microsoft Windows Shortcut 'LNK' Files Automatic File Execution Vulnerability (BID 41732).
W32.Stuxnet!lnk is a detection for .lnk files created by the W32.Stuxnet worm.
Bloodhound.Exploit.346 is a heuristic detection for files attempting to exploit the Microsoft Windows Shortcut 'LNK' Files Automatic File Execution Vulnerability (BID 41732).
So, as we see these above Threats appears when there are open vulnerabilities on the machines.
In your case, I would suggest the below Plan of Action:
1) Make sure ALL Computers are installed with Symantec EP with latest / updated with virus defintions.
2) Install ALL Latest Microsoft Secuirty Patches / Sevice Packs on ALL machines.
3) Make sure ALL the client machines are using the Latest Vendor Patches installed.
4) Disable Auto play with GPO
http://support.microsoft.com/kb/953252
5) Disable the System Restore with GPO
http://support.microsoft.com/kb/283073
6) Disable Scheduled Tasks with GPO
http://support.microsoft.com/kb/310208
7) Enable Security Auditing with GPO
http://support.microsoft.com/kb/300549
8) Incase of any shared / mapped drives present, make sure these are password protected.
9) Scan ALL the machines...
Hope that helps!!