If you have a license for the software and you still have an old copy, sure.
With that said, the 3.X release tree is like 8 years old. It’s not supported and there are known security issues that were mitigated in later releases. In many cases the software will not run on modern servers as the software was not developed or tested against modern web servers.
Finally while most web browsers will be backwards compatible, there might be issues in the UI related to that as well.
Basically you’re locking yourself into a version of software that is not keeping up with servers or your users so you run the risk of being stuck with no support including difficulty upgrading to a supported version later.