GM does not license the name Dexron 3 anymore hence you cannot see it anywhere now. The most you will find is Dex/merc, which is perfectly fine to use.

I personally have used tractor supply co. ATF and also Brad Penn ATF III-H, both of which were equally good to use. For budget ATF, you can use Same's Club ATF, they also were selling the Penzzoil ATF for about 30 dollars shipped free. Used that in a 2002 chev impala (all 12 qts flush) and it is also running great.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)