REDUCE: minimize how many animals are used.
REFINE: use techniques/procedures that minimize distress and pain to animals.
REPLACE: substitute animal models with non-animal techniques or lower organisms.
Adapted from: Russel, WMS and RL Burch (1959). The Principles of Humane Experimental Technique. Universities Federation of Animal Welfare: England, 238p.
More info:
National Centre for the Replacement Refinement and Reduction of Animals in Research