I have some that i bought in 2000~2001, used some of it the other day on a AM2 rig nothing has melted as of yet. take that as practical reference if you wish. Just make sure its not to runny and you apply a good application of it and it should be fine.
The only issue I would see was if it was stored on its side or point-up. AS's website says that the compound can separate over time, and it should be stored point down to keep the thin stuff near the back of the syringe.
Or you could always centrifuge it!
OR... Go out and buy some new stuff. It only costs like $10.
I have access to a spin dry station used for drying silicon wafers and such. Old Bessy cranks up to about 6.2K RPMs before she becomes to violent from vibrations .
I had some old AS5 that was like 2 years old and the stuff was so thick now I couldn't even spread it anymore. I would just invest in some new thermal compound.
AS5 and Ceramique are better than AS3 anyway. I had some Shin Etsu X-23-7762 that came with my cooler (sig) and used it last time I reapplied thermal paste. I just took my cooler off to clean it up (very dusty) 2 days ago and noticed that it didn't spread as well as AS Ceramique or AS5 so I cleaned it all off and reapplied AS5 (since my Ceramique is at home). Temps are a couple of degrees C cooler and I was able to give the old 3700+ Clawhammer more volts to boot!
Bottom line - use newer/better TIM and don't waste your time on the old AS3, especially with that nice, new cpu.