Since a rhombus is symmetrical, if you have two points already you should be able to get the other two by mirroring.
In this case, take (0,3) and mirror it horizontally (not to be confused with vertically mirroring; there is a huge difference.) To horizontally mirror this point, fold your paper along the x-axis. You should get (0,-3) as your new point.
~The reason we mirrored (0,3) horizontally is because this point is on the positive y-axis, and we want to see where it'll be on the negative y-axis. Horizontally mirroring just helps us do that.
For your next point, take (3,0) and vertically mirror. This time, fold along the y-axis. Your new point should be (-3,0).
let me know if you have any questions:)
