First of all, the Earth's orbital velocity around the sun is approximately 30 km/s not 3km/s.
Your suggestion does not make any sense, since gravity is created by the mass of a planet, not by waving a magic wand.
"Rocket Scientists" have already figured out many ways to make use of gravity from various solar system bodies for designing interplanetary trajectories, or just changing orbits. Since it seems you are unfamiliar with these, playing Kerbal Space Program could be a good start, since while it ignores perturbations including third body gravity (meaning no Lagrange points) it still can let you experience the Oberth effect, and gravitational assists.
If you really want to get serious about studying this, you can skip the game and take a class or pick up a textbook on orbital mechanics. There is also plenty of good information on the Q&A section of this site as a starting point, for example this thread:
https://forum.nasaspaceflight.com/index.php?topic=39788.0