Repair the quantifier-relative error: The orchestra has twenty players, three of them who joined this year.
The orchestra has twenty players, three of whom joined this year.✓Bonne réponse
The orchestra has twenty players, three which joined this year.
The orchestra has twenty players, whom three joined this year.
The orchestra has twenty players three of who joined this year.
Explication
Three of whom creates the standard non-defining quantifier relative and links the subgroup directly to the players. Keeping them before who leaves two competing ways to represent the same people and does not form the intended structure.