You can trade 3 copies of the exact same player for a different player of the same rarity (level 1).
This means that if you have 3 Messis, you can swap for any other Black Ball, like Ronaldo. It's important to note that you cannot trade for legends.
Also, keep in mind that they need to be the exact version of the player for that to work - you can’t mix featured player versions with regular versions.
To do so, go to the CONTRACT / TRADE screen and pick the duplicate player you want to trade.