Geometric model of perfect ciphers with three cipher plaintext values

In this work we deal with the problem of describing Shannon perfect ciphers (which are absolutely immune against the attack on ciphertext, according to Shannon) when cardinality of alphabet of cipher plaintext values is equal to three. It is shown that there is no minimum by inclusion perfect ciphers with five or six encryption keys. The number of minimum by inclusion perfect ciphers with seven and eight keys are determined. Examples of minimal ciphers with respect to inclusion are built.