Short answer

Available Margin represents the total amount of funds that can be utilized for trading on a particular day.

Available Cash refers to the closing balance from the previous day’s, carried forward.

Used Margin is:

The net funds used for executed equity intraday, F&O positional/intraday trading, and delivery orders.

The funds blocked for open orders that have not been executed yet.

The amount displayed as the used margin when selling shares or holding open F&O positions.

When positions are squared off, the used margin is credited to the Available Margin.

If no additional funds were added on a specific trade date and trading activity took place, the Available cash can be calculated as the sum of the Available Margin and Used Margin.