Antifreeze
 

I bought a 99 silverado 3 years ago in condition nothing wrong with it. I live in south Alabama were having a cold winter so I checked the antifreeze. And the person that had the truck before me put the green type in. When they should've put the dexcool. Besides flushing and changing to the dex. Will there be any problems that might develop with the engine or cooling system that I can fix ahead of time or is it too late.

There will be no problems.
Once green coolant is mixed with dexcool it is no longer a long life coolant and must be changed at two year intervals.

A lot of folks used to swap out dexcool due too it plasticizing intake gaskets if not changed on time.

Unless you are ready to do a system flush to remove all the green coolant; do fluid changes every two years.

Which green coolant? Even Prestone mix with any color good for 5 years stuff?

historically green coolant in north America is silicate 2 year coolant.

There is no way to identify if the system contains an organic acid coolant or silicate based coolant. Imo it should be treated as a 2 year coolant.

You can use the green coolant instead of dexcool, but it is not recommended to mix them because it will knock down the warranty life of the dexcool to the life expectancy of the green one.

Keep the green conventional coolant.