what size propane back-up generator is needed to run
The 3 hp pump full load amperage should be 230 V 3910 VA x 125% since this is the largest motor 4887.5 VA.

The 1/2 HP would normally be 115 V 1176 VA.

3 refrigerators / freezer total 4320 VA for a total of 10,383 VA.  I may be a bit over on the fridge/freezer size since I used 80% of 15 amp x 3.

I would not go lower than a system that had a peak rating of 10 kW but a continuous wattage closer to 7.5 kW should work.

Watch out.  Some generators list their peak rated value and some list  continuous rating trying to make a smaller unit look like a deal.

I have a 8.5/13? (peak/continuous kW) that runs 2 wells (3 hp &amp; 1.5 hp both 230 V) 2 fridge/freezers, all the lighting in the house and barn and power to run the wood stove fan with some reserve.