Although interpretations of these three words vary a bit, here are their basic definitions:
Reduce: Researchers should Find a way to lessen the number of animals needed to explore the question. This can be done in a myriad of ways, such as good use of statistical analysis and experimental design, including calculating minimum sample sizes that are still big enough to discriminate among the groups, using a more complex experimental design, and using animals with limited genetic variation to allow for a smaller sample size.
Replace: This means experimenters should use non-animal methods (often technology) instead of animals. Examples of this include in vitro studies of human cells and tissues. A fuller list of examples is here.
Refine: This means to adjust the experiment so that it inflicts less pain and suffering on animals than it would otherwise cause. From the IACUC handbook: “Refinement of technique to reduce or eliminate unnecessary pain and distress in study animals is the most commonly practiced of the 3 Rs, although it is not always recognized as one of the applications.” While this originally meant anesthesia, it also includes analgesia (localized pain relief), or just finding less invasive ways to achieve the same result. This can also mean enhancing animal welfare through refinements in social housing and enrichment.
The phrase “Replacement Alternatives” covers all 3: The word “alternatives” is often used to encompass the Three Rs. CAAT (Johns Hopkins) defines alternatives as “methods that reduce animal use, replace whole animal tests, or refine existing tests by minimizing animal distress."
History of the Three Rs:
1959: the Three R's were first proposed by UK scientists William Russell and Rex Burch.
1960s:: Many advances in non-animal techniques came online, making a discussion of alternatives realistic.
1978: David Smyth publishes Alternatives to Animal Experiments
1982: Johns Hopkins opens its Center for Alternatives to Animal Testing (CAAT).
1985: Congress amended the Animal Welfare Act (AWA) to formally incorporate the Three Rs in several ways. Perhaps the most significant of these was Congress revising the AWA to “require research facilities establish Institutional Animal Care and Use Committees ("IACUCs") to implement the Three R's.”
1986, the US Congress Office of Technology Assessment (OTA) published a report on Alternatives to Animal Use in Research, Testing and Education.
1997: NIH Director establishes the Interagency Coordinating Committee on the Validation of Alternative Methods ("ICCVAM").
2000: Congress makes ICCVAM permanent. In doing so, it wrote the Three Rs into the Animal Welfare Act. It’s critical to note that one of ICCVAM’s main purposes is to "reduce, refine, or replace the use of animals in testing, where feasible.”
Everyone’s “Committed to the Three Rs”, at least in theory
Governments, corporations, trade groups, and even companies that breed animals to die in medical experiments tout their adherence (in principle) to the Three Rs.
“Vanta Bioscience is committed to the 3Rs (reduction, refinement and replacement) of animal based test methods by offering validated alternative toxicological methods."
“[GlaxoSmithKline] remains committed to the 3Rs”.
“Charles River is committed to the 3Rs (Replacement, Reduction, and Refinement) and continues to apply these principles in preclinical toxicology." Irony alert: Charles River’s main revenue is from breeding and selling animals to labs for experiments.
“The FDA is committed to the 3Rs, by reducing, refining, and replacing the use of animals in drug development."
The “American Association for Laboratory Animal Science Foundation (which The laboratory animal science community) is committed to the 3 Rs: replacement, reduction and refinement.”
NIH Funding attached to the Three Rs:
Anyone that takes funding from the NIH has to comply with two documents that incorporate the Three Rs: (1) The Public Health Service (PHS) Policy on Humane Care and Use of Laboratory Animals and (2) NIH’s Guide for the Care and Use of Laboratory Animals.
The three Rs are undeniably a key part of federal policy for biomedical research and toxicity testing, and the U.S. Congress has repeatedly stated the importance of these principles. But while many parts of our government are working hard to achieve reductions in animal use (FDA, for example), many other entities aren’t taking this seriously. For example, the number of primates being held captive and experimented on hasn’t gone down, but actually just hit an all-time high.
We need to correct this trend and ensure that fewer and fewer animals are subjected to cruel and uneccesary tests and more human-relevant non-animal alternatives are developed. You can help this become a reality. Please tell NIH to request more funding for alternatives.
 ICCVAM Authorization Act (42 U.S.C. 285l-3)
 ICCVAM Authorization Act (42 U.S.C. 285l-3)