Explanation:

The three-column cash book is a bookkeeping tool used to record cash and bank transactions. It consists of three columns: the cash column, the bank column, and the discount column. The purpose of these columns is to record different types of transactions.

The three columns of a three-column cash book are:

1. Cash column: This column is used to record all cash transactions, including cash receipts (money received) and cash payments (money paid). It is used to track the inflow and outflow of cash in the business.

2. Bank column: This column is used to record all bank transactions, such as deposits into the bank account, withdrawals from the bank account, and any other bank-related transactions. It is used to track the movement of cash through the bank account.

3. Discount column: This column is used to record any discounts given or received by the business. Discounts can be given to customers for early payment or received from suppliers for prompt payment. The discount column helps in tracking these transactions separately.

The column that is not a part of the three-column cash book is:

Petty cash column: The petty cash column is not a part of the three-column cash book. Petty cash is a small amount of cash that is kept on hand for making small payments or reimbursing minor expenses. It is usually managed separately from the main cash book and has its own separate record.

The petty cash column is typically found in a petty cash book or petty cash voucher system. It is used to record petty cash transactions and track the balance of the petty cash fund. The petty cash column is not included in the three-column cash book as it is a separate entity.

In summary, the petty cash column is not a column of the three-column cash book. The three columns of the three-column cash book are the cash column, the bank column, and the discount column.