Could your question be phrased as below?
I need 3.3V at 3A, and have a 12V/3A power supply. I have some fixed-voltage 3.3V/1A linear regulators. Can I put them in parallel and get up to 3A at 3.3V from the combination?
If so, then it's highly unlikely. The first problem is that your regulators will not output exactly the same voltage; whichever one has the lowest voltage (even a small amount is enough) will attempt to supply all of the required 3A current. It will overheat and fail quickly.
The second problem is that your regulators are rated for 3.3V/1A under great conditions. First, that they don't have to dissipate too much power, and second that they are heatsinked enough to keep them near room temperature. It's unlikely that you'd even achieve 3.3V/1A from the top circuit. Your regulator will attempt to burn off (12V-3.3V) * 1A = 8.7 watts as heat. In a SOT-223, that's a lot of heat to get rid of somehow. If you look in the datasheet, you should find a table or graph that indicates what current ratings are possible at various junction temperatures. And you should also find a case dissipation rating (degrees per watt) that will help you find the difference between ambient temperature (what your heatsink can manage) and junction temperature at a given amount of dissipated watts. Then, if you're specifying a heatsink, you'll need to find out how many watts it can transfer to the air depending on various ambient temperatures.
Sound like a tough problem? It is...it's an engineering problem, not just plugging parts together and expecting them to work. You can significantly reduce the amount of risk/worry by choosing a different regulation solution:
* Find a premade regulated power supply with the desired amperage. Everything is already handled for you.
* Change the power supply to 3.3V + regulator overhead and not much else, and use a linear regulator that can supply at least 3.3V. This will greatly reduce the amount of heat dissipated from the regulator.
* Instead of linear regulators, choose a switching buck regulator design. This will have much greater efficiency, so not only is heat not a big problem, but you also aren't uselessly wasting twice the power your circuit actually needs. There are simple switching regulator designs like the LM2576 that don't require many external parts.
* Find a premade switching step-down regulator board or DC-DC converter that can supply the voltage and current you need.