Dex 6 supersedes Dex 3 and can be used in all prior Dex applications where 3 was specified or accepted.

If you read the data sheets on Dex 6 vs Dex 3 you wouldn't have any doubt as to which one to use...though EITHER will work in your application.

The only downside to Dex 6 is the price. it's about 2 to 3 times that of Dex 3