I want to switch over to synthetic, can i use the dexron 6 or does any one know where i can find a synthetic dexron 3?

No point, it won't do any good in a trans that old. Don't use Dexron VI.

Any auto parts store will have a standard Dex/Merc labeled fluid that meets the older Dexron II, and III requirements. Most of the "Multi Vehicle" labeled fluids meet Dexron III spec as well.

Ok i just have about 4 gallons of the stuff and, i dont have any other use for it, so i figured could throw it in the subaru if i wont do any harm. Thanks

Supposedly you can use Dexron 6 it but in my opinion it is not the optimal fluid for that application. For one reason Dexron 6 is thinner to improve fuel economy.