Planetary gears slip transmission
I'm going out on a limb but if you only maintained your internally-geared hub just once, three years ago, then its probably excessively worn due to poor maintenance.  Also the oil is likely carrying metal flakes which are doing continuous damage to the gears.

IGH are low maintenance but not zero-maintenance.

An oil change right now might be enough to bring it back but excessive wear will likely have shortened its overall life.  Depending on wear, you might benefit from a new 1/8" chain and new cog too.

If you can't or won't do maintenance yourself, then simply book it into the local bike shop for work annually and pay for someone else to do it.