nfs3 mount option

Post your entire /etc/fstab file + list of all installed packages.

Linux is very smart. If for example you try + mount using nfs3 + your Kernel only support nfs4, then you'll end up with an nfs4 mount.

My guess is you'll have to run a very old version of Linux to have nfs3 support, as nfs4 became mainstream in 2012ish.

If your Kernel is built with nsf4 (which will be every... probably post 2013-2014ish Kernel this will be impossible.

That said, installing a downlevel Kernel with nfs3 support is a bad idea also, because this will likely be a v2.6.x Kernel which has known exploits, so eventually you'll end up hacked.

My suggestion is start with Ubuntu Aardvark which will release in a few days, which will have Kernel-4.13 available.

Then follow one of the many guides for correctly mounting old nfs3 clients on a modern nfs4 machine.

This will likely resolve your problem.

Also, if your data is important/sensitive, best completely move your files of your nfs3 system as soon as possible + retire this system, as like I said above, the Kernel is likely hackable.

What is the functional issue that you nfs3 would do that nfs4 does not provide?

add to the mount option (-o)

mount -o vers=3

or

mount -o nfsvers=3

though as mentioned depends on what distribution and version you are using; may not be supported any longer

Arnold has a good question. Normally, nsf4 is backward compatible with nfs3, so maybe stating what problem you're having accessing your nfs3 machine from an nsf4 machine might help.