I am returning to USA soon and will return wilth a 1 year multi-entry immigrant ) visa based on marriage to a Thai. When I return I will have 3 months with an extension remaining on a 3 entry tourist visa. Can I use the tourist visa’s remaining 3 month and then begin the immigrant O visa. Of course I am assuming I cannot, but just checking the possiblitly here.possibility

You cannot.

The embassy or consulate may cancel your existing visa before they issue the new one.

Remember also that the new visa will expire one year from the date of issue. So not using it when you enter would gain you nothing. You can get 15 months from a multiple entry visa by doing an entry just before it expires. Even getting the visa to early causes you to loose days spent in the country.