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The only way that having 3KW inverter is practical is to raise the DC-link voltage
Stack a load of 12V battery's in series and you will drascically reduce the current (which is the main problem here) this will increase the efficiency since there will be a vast reduction in copper-losses
Yeah getting "schematics" are the least of your problem. Handling 300 amps is serious business. The traces alone would have to be incredibly thick. Doing high freq switching at this current level, also a trick.
And a car battery isn't going to be happy putting out 3KW. A single deep cycle battery's voltage will drop substaintially under loads this high. It will drop even more once the battery is not fully charged. You'd need many large deep cycle batteries in parallel to sustain this.
What are you trying to do ultimately?
It's not impossible, but not very practical to power, and extremely difficult to build from scratch.