AC Power Splitter and Regulator
probably the easiest way to make that would be to take a desktop 12V 5A power supply and splice a 5A buck module into the output cord

Sellers of cheap buck modules tend to over-estimate their capability by a bit - that's why I reccomend a "5A" modules to suppyk your "3A" need.  a search for "XL4015 5A" should get you some examples of the type of module I mean. cost is about $1 on aliexpress or banggood

If you can live with a lower power level a hard drive testing power supply can typically do 12V2A and 5V2A simultaneously