Did God promise the Paradise only for one out of the three groups of people who have initially accepted His religion?  

Verses 32-33 of Chapter 35:

Then We have given the Book for inheritance to such of Our Servants as We have chosen: but there are among them some who wrong their own souls; some who follow a middle course; and some who are, by Allah's leave, foremost in good deeds; that is the highest Grace. Gardens of Eternity will they enter: therein will they be adorned with bracelets of gold and pearls; and their garments there will be of silk.