lm7812 configuration
The basic method of boosting the output current of 78XX voltage regulator is by adding an external PNP power transistor, and that is readily described in most of 78XX-series data sheets ..
Now, you try to use TIP122, which is NPN power darlington ..
Of course, it is possible, and all what you have to do is to convert this NPN configuration to PNP by adding small signal PNP transistor to your TIP122 - see attached picture ..
Also, make sure that the input can supply at least 3A @ U(unregulated)>16Vdc ..
Regards,
IanP