With the x and z force is being applied every frame to create the movement. With the way you are applying force to y, you would need to hold the button like you do the movement keys. If you want it to jump on press, remove the y variable from force.set() and use impulse for it instead. Impulse immediately applies the desired force to the rigidbody, while using force applies a small amount over time. The applied impulse would also have to be greater than the gravity in scene (default -9.8).
For me it does exactly what I said applying force would do, the fps character starts to float while I hold space. One thing I may suggest to fix this, is to check if the player is grounded.
This is how I would go about it: