Connecting Sub Ground to Main
Thankfully, fixing this is fairly simple
The good news is that pulling an 8AWG copper grounding conductor from the subpanel to the main panel should pose no trouble, as you have plenty of fill to spare in your conduit for that fourth wire.  In particular, Schedule 80 PVC in that size is no issue at 320mm² of usable fill area, which is ample space compared to the mere 211.92mm² of fill your feeder will take up once you add the 8AWG grounding wire to it.
The bad news is that you'll want to pull the existing wires out, pulling a line in at the same time, then use that pull string to pull the new bundle through.  (You should be able to reuse the existing conductors provided they're in decent shape, though.)
Other notes: that 80A breaker can be bumped up
Note that there's no reason to put this feeder on an 80A breaker; 3AWG copper wires with an 8AWG ground can handle 100A when landed on 75°C terminations, which modern distribution equipment will have, as a rule.  So, you can swap that 80A breaker for a 100A breaker. (Square-D doesn't even make an 80A panel, by the way; they have some tiny &quot;spa boxes&quot; rated for 70A or less, but anything larger will have 100A or 125A busses on it.)