Hi Coopz. Those extra holes are for the bead locks which look similar to this.

They were fitted to these bikes to stop tyre creep on the rim due to the enormous power these Rh's developed.
Yours have obviously have been removed by someone frustrated by them when trying to fit a tyre onto the rim. Normally, the holes are then plugged with a dome headed bolt to fill the hole left.
Sometimes, they fit tubeless valve stems to all three holes and run the rear wheel tubeless, though I'd feel better with a good 'ol tube in there.