1. The time of expiry is 3.30pm. However, the expiry price is the VWAP over the last 30 mins (from 3pm to 3.30pm).
2. Assume in the morning on expiry day, the cash is at 100 and the futures are at 101. You buy the cash and sell the futures. At 3pm, the cash and futures are both at 100. You have two options: one, is to sell the stock and buy back the futures simultaneously. Second is to sell the stock over the last 30 mins. Since the futures will expire at the underlying stock price, you only have to ensure that you sell the stock at the closing price (or higher).
3. VWAP is Volume weighted Average Price. Calculated by multiplying the number of shares traded at each price by the price and dividing this by the total number of shares traded.