Alice and Bob decided to perform a secure communicate using symmetric crypto system. They decided to use Data Encryption Standard (DES), but they understand the key size of 56-bit for DES is...

Alice and Bob decided to perform a secure communicate using symmetric crypto system. They decided to use Data Encryption Standard (DES), but they understand the key size of 56-bit for DES is too-short. Alice and Bob decided to do the encryption/decryption three times using three different DES keys to increase the overall key size for the encryption/decryption operations. Explain how Alice and Bob can increase the effective key size of the operation to 168-bit using three different keys?