if I needed to drain the rad fluid to replace a thermostat AND the fluid was relatively fresh I would reuse it.
The OP said the rad fluid was 3 years old.
As others have noted there was no talk as to why the fluid was drained, has slot to do with the answer.
But 12.00 for a gallon of 100% rad fluid (@ 50/50 is 2 gallons) is cheap insurance.
Trans fluid I would always replace.
