For 100% efficiency (no friction, etc.):
FD=Ld where F is the applied force (in Newtons) and D=3m the wheel diameter, and L=850kg=8339N is the load and d=0.25m the axle diameter. (We could have used the radii instead of the diameters. Mechanical advantage is the ratio of the radii (or diameters)=3/0.25=12:1. This means the wheel can lift 12 times the load on the axle in a frictionless system.)
So 3F=8339×0.25=2085, F=2085/3=694.88N approx.
However, the efficiency is 87% (less than 100%) which means that the force must be correspondingly greater to compensate for friction, etc.=694.88/0.87=798.7N approx. (equivalent to about 81.4kg weight.)