I cannot find one that is designed for the A3000 but I have seen some soft case that looks like this:

However, while these are affordable, and should be reliable enough. It is worth noting that your camera was not designed to be use under water. It has small buttons that can be extremely hard to press when under water, and dials are not exactly easy to turn under water when wrapped in a thick layer of rubber.

If you see yourself diving a lot and doing a lot of underwater photography. You may want to get a water-proof point and shoot camera. They are priced like any other point and shoot camera, and is relatively affordable.

As a bonus you will not have to worry about it getting water damaged. You obviously do not have to pay 40 bucks for a thick plastic bag. Finally the buttons are usually slightly larger, or have some helpful design, for easier under-water operation.

You will not be changing memory cards under water, but it is still a very good idea to buy waterproof memory cards (professional cards like the Sandisk Extreme Pro series are waterproof). Last thing you want is dropping a memory card into the water and have all photos dissolved.