Pololu has 5volt regulators that are about the same size as an 7805, and can be mounted as an 7805.
The board from a cigarette lighter USB supply is about the same size, but might not be as neat.
What's the reason you are using 3volt relays if you have 12volt available.
12volt relays (~30mA) can be powered directly from the car battery supply.
The controller used to turn on and off the relays is a 3.3v system.
What is your project?
If you do design your own PCB and put a DC-DC converter on it, look carefully at the DC-DC IC datasheet, as it will have some specific requirements with respect to layout and PCB track width.
A DC-DC converter is a high frequency device and can be very sensitive to component layout.
The project is working perfectly fine separately. I am using some sort of DC-DC converter I got of amazon. Its just used to control a few Dc motors based on readings.
I was copying the data sheet for the DC-DC buck converter. I was looking up trace recomendations for what amperage and was just following those. The area its in should have plenty of clearance around to oversize the traces.
I'm still not sure what exactly I am going to use. I really dont need much power but I would like for a good known trusted schematic that is easy to implement, reliable and doesnt get too hot.