How do I increase 3A fixed output current of a buck converter?

Basically I am making a solar inverter without any battery or charge controller that will directly convert the dc output coming from solar panels (6 connected in parallel) into 220V AC.I am using solar panels 50W each, having an open circuit voltage of 20V and the voltage varies between 15-20V during the entire day provided a minimum amount of sunlight is there. Next, I am using a 12V buck converter circuit using an LM2576 (here is the datesheet of lm2576)and few more components to get a stable output voltage of 12V out of the panels. Now this 12V DC is fed to an inverter circuit which converts it into a square/modified sine wave 220V AC at approximately 50Hz. But, I am not getting desirable power output. From 6 panels, all I am able to power is a 45W LED bulb along with a small 3W LED bulb.

Probably, one problem is with LM2576 buck converter IC. This IC although providing a constant 12V output but it is rated at a 3A fixed output current. And I think probably this is the reason why we are unable to drive more loads. Is there a way to amplify current in this case? Or something else should be done which I am missing here ?

1. Are your panels rated for a parallel connection?
E.G.: do they have protection diodes build in?
2. 50W is a figure from the seller based on maximun sunlight at noon with the panel aligned correctly.
This means on a normal day you might only get about 30W if lucky.
3. Using a limited buck converter not only introduced a lot of interference but as you noticed 3A just won't cut it.
Better use a solar charge regulator rated for 30A that offers a direct output for a load and not just the connection for a battery.
If you really want to get good use I recommend to connect a decent battery anyway and to drive the inverter from it - this will prevent your stuff from blowing up if the clouds come while you need a full load.

Even if you assume no losses anywhere a 12V/3A input for the inverter will only have less than 40W and you won't get more on the output side.
Might be easier to modify an old computer UPS for your needs....