What is the difference between dextron fluid and atf 3?

Answer 1

Dexron 3 is just a reformulated version of Dexron. There is no such fluid as "Dextron", although Dexron" is commonly called that because it is easier to say.

Answer 2:
You won't like this answer, but the real answer of what is the difference, is "Trademarked Marketing" -- simple as that, Dexron is a trademark of General Motors, ATF+3 is a trademark of Chrysler (Dodge), and you must pay them a royalty if your fluid bears 'that name' or 'trademark'.

The chemical reports of these two fluids, are:

DEXIII:
Viscosity: 201 (indexed): @40C - 34.70 : @100C - 7.70
ATF+3:
Viscosity: 200 (indexed): @40C - 33.70 : @100C - 7.60
To Compare;
Redline Race ATF:
Viscosity: 177 (indexed): @40C - 53.70 : @100C 10.00

In your transmission, the difference (if you could even tell) is when it shifts & how hard it shifts. Just stick with a good name brand and your car won't know the difference.
*NOTE: Dexron VI is NOT compatible with any other fluid on the market, do NOT attempt to use Dexron VI in a transmission that does not call for it, or vise-versa.