Do FDA require that when validating a process using 3 validation batches that all 3 validation batches are placed on stability?
YES. Samples from all three batches should be kept for stability studies.
Plz reference of this rule?
You can see a clear regulatory requirement for FDA ragulated industries, within the CFR21:
STABILITY TESTING (21 CFR 211.166)
“…it is imperative that stability studies are not limited only to initial production batches but a portion of annual production batches be the subject of an ongoing stability program.”
((Considering that the initial production batches should be Validation batches))
Additional guidance from the ICH Q1A:
STABILITY TESTING OF NEW DRUG SUBSTANCES AND PRODUCTS Q1A(R2)
Data from formal stability studies should be provided on at least three primary batches of the drug substance. The batches should be manufactured to a minimum of pilot scale by the same synthetic route as, and using a method of manufacture and procedure that simulates the final process to be used for, production batches. The overall quality of the batches of drug substance placed on formal stability studies should be representative of the quality of the material to be made on a production scale.
I hope this helps