Basic Utility Theory states that a consumer will continue to buy product whilst Marginal Utility exceeds price
The Marginal (Total) Utility of 1 camera = 300 + 400 + 200 = 900
The MU (2 cameras) = 280+ 360 +160 = 800
The MU (3 cameras) = 240 + 300 + 120 = 660
The MU (4 cameras) = 180 +220 + 100 = 500
The MU (5 cameras) = 80 + 100 + 10 = 190
Clearly 3 cameras Marginal utility (660) > Cost (500) but if you add another (4th) camera the Marginal Utility reduces to only 500 which equals the cost. As it does not EXCEED the cost, the 4th camera will not be bought.
Answer 3 cameras will be bought
This information should now be sufficient for you to answer your other two questions by yourself.