I am looking for a specific cryptosystem with the following properties:
Suppose a User (Say $User_1$) encrypts a message $M$ with the key $K_1$ (the resulted ciphertext is $C_1$). Another user, say $User_2$, encrypts the ciphertext $C_1$ with another key $K_2$ (the resulted ciphertext is $C_2$). Now, a third user (e.g., $User_3$) should be able to decrypt $C_2$ with the key $K_3$ to achieve the message $M$.
$C_1 = E_{K_1}(M)$
$C_2 = E_{K_2}(C_1)$
$M = D_{K_3}(C_2)$
Note: The keys may be related to each other, but they should be three different keys.