I paid for dinner with 3 of my friends.
And at the end of the day, these 3 people send me Rs.500 each. They sent me the money via UPI bank transfer (say, using phonePe or BHIM).
Will this amount (Rs. 1500) be taxed?
No, this amount is not taxable. For it to be taxable it should be more than ₹ 50,000 in a financial year.
Even then if you can establish that the transfer of funds was not a gift and more of convenience, there is no tax applicable.
In addition to the reasons provided by Dheer, the amount is not taxable for other reasons.
Most importantly the payment is not income to you. It's payment to a restaurant for a dinner, a payment that happens to have been paid through you. You might consider yourself the (unpaid) agent handling the transaction for him, or you might consider it a very short term interest free loan. Neither are going to attract tax.
Secondly the tax office is not interested in amounts as low as a single dinner. That's not a legal reason but a very practical reason. If it was a larger amount the tax office might require you to document the transaction and explain why you were processing it for your friend. But for an amount as low as a dinner they are simply not going to care.