Can Gm dexron 3 or dextron 3G ne used as power stering fluif of a subaru car?

Yes — GM Dexron III / Dexron III-G ATF can generally be used as power-steering fluid in many Subaru vehicles, but it depends on the model year.

Typical Subaru requirement
Most Subaru vehicles (especially from the 1990s up to around 2010) specify:

Use Automatic Transmission Fluid (ATF) meeting Dexron III specification in the power-steering system.
So if your cap or manual says ATF or Dexron III, then Dexron III / Dexron III-G is appropriate.

 
When not to use Dexron III
Some newer Subaru models (mainly 2011+ electric power steering or CVT-based systems) no longer use hydraulic fluid or may require:

Subaru ATF-HP
Subaru ATF-CVT
Or they have EPS (electric power steering) with no fluid at all.
For these models, Dexron III should not be used.

 
How to confirm
Check:

Power steering reservoir cap — often says “Use ATF” or “Use Dexron”.
Owner’s manual — fluid specification section.
 
Recommendation
If your Subaru cap/manual indicates ATF Dexron III:

Using GM Dexron III or Dexron III-G is safe and correct.