The 3-subset meet-in-the-middle (hereafter shortened MITM) attack is a variant of the generic meet-in-the-middle attack, which is used in cryptology for hash and block cipher cryptanalysis. The 3-subset variant opens up the possibility to apply MITM attacks on ciphers, where it is not trivial to divide the keybits into two independent key-spaces, as required by the MITM attack. The 3-subset variant relaxes the restriction for the key-spaces to be independent, by moving the intersecting parts of the keyspaces into a subset, which contains the keybits common between the two key-spaces.