Hero cards are the cards that are not affected by things such as scorch or weather cards and they have their attack number in a black circle with an orangy outline. These cards are normally aquired through beating notable characters in gwent matches.
All you have to do is aquire 3 of these cards and restart a gwent game until you get all 3 in one hand and win a round. Hero cards are generally very good so winning a round shouldn't be too difficult
If you are having trouble finding hero cards then i would recommend using the guide for the card collector achievement to find them. Be sure to go to the card collector achievement under the the witcher 3 wild hunt and NOT the witcher 3 wild hunt game of the year edition as it has no guide