motor controller requiring a battery in parallel with DC power supply
The reason that they suggest a battery is efficiency (And back-EMF protection, cheers Tony).

The battery can store the energy generated from regeneration, and then it can be used when you next demand power. You could configure a capacitor bank for the same thing, however the quantity of stored charge would require quite a large capacitor bank and would be expensive. It also allows the motor to draw surge current.

A resistive load will waste a great deal of energy, as commented by Brian, 800W is quite a lot, and if you were working in regen mode for a long time it could burn whatever load you have designed (unless you accounted well in terms of cooling)

And YES, you could use three 12V SLA (lead acid) batteries in series as you suggest (if they are lead acid), be sure they can handle the required regen current though. (This is what I would do if you have them availiable - they cope well with a floating voltage on there all the time :-) )