You'd still have to program the game yourself. A 3d Engine a set of routines that at a minimum, draw 3D graphics. Sometimes more will be added like a collision system or more. You might need one or two assembly language routines for speed. No, you would not need a 3D engine.
Source code is what the programmer types into the computer.
A program called a compiler reads the source code and turns it into a program for the computer to run. Because 3D programs are so expensive could you use C as your graphics program. It's not easy to use but it is free. I depends on if I could find an engine that fit the game I wanted to make. Generally though I would probably write my own.