These are the Gringotts Coins: Galleon, Slicke and Knut.
All the designs have the size the should have in real life, although you may have to scale it for having a better printed coin.
I suggest to scale all of them to the Galleon's size (40.6 mm of diameter and 2.5mm of heigth)
All coins are flat from behind. I recomend printing each side of each coin separated and then glue them. Beware you may have to resize the back of the coin if you haven't resized the sickle and knut to the galleon's size.
I recommend printing the coins in a neutral colour.
After printing I used spray over the coins and I got the result shown in the picture. I think this is the best option to get the best result