I am sorry, but expecting 3 seats to be blocked is just a little bit entitled, if you are a Plat. You paid for econ, you might get one seat blocked but I don't see what would entitle you to get an entire row. What if everyone would ask to have all seats blocked? What if another flight was delayed/canceled and they needed to rebook those on your flight?
If anything, the gate agent should have just denied your outrageous request.