This is a linear regulator, 7805 can regulate the output at +5V at 1A of maximum current.
That regulator, alone, can't supply 3A.
Of course, if you want to continue in this way, you need to change the line main transformer for more power and then use different schematics for regulate the output.
You can still use 7805 like regulator and you can add a power transistor, for example, to provide more current at the output; but this is only the one way to do this power supply...
I invite you to think about the main transformer specification in that schematic, read the inpunt voltage... 230V (110V)..
It's strange because there aren't any switch selector for different windings of that transformer so... with one seconday and one primary the output current and voltage of that transformer will be only one... impossible to work well in a different input voltage.
So first of all you need to know, or decide, if you need the power supply for multiple input voltage or not...
Then i suggest to think about the efficiency of this kind of system for regulate the voltage at 3A... remember, this is a linear approach and 5V x 3A = 15W
Think, make your decision and... we are here