I would not recommend a 3 blade prop for any belly lander. A 2 blade prop will be far less prone to break while landing.
If you are running a 6x4x3 now you should be able to run a 7x4 without any issues. It's possible you could use a larger prop.
When using a new prop size it's always a good practice to do a bench test. Run the prop full speed for 10 seconds, stop and test the temperature of the motor, ESC & battery by touching them. Warm is OK, but if they are so hot you can not hold them, stop, use a smaller prop. If it passes, try a 30 second run, then a 60 second run. If is passes you will be OK, you could even try a larger prop. Keep in mind that a prop that worked when is cool, 70's, may over heat when it gets hot out, 95+.