What do you expect to do with all the other points (vertices)? It appears you want to raise one point in an irregular object. It will have to deform somehow. You know you want two other points to stay put, but there are many other points (vertices) on the object that have to move one way or the other. Or would you like to rotate the whole object or the top face about an axis containing the two fixed points, and keep the surface in one inclined plane? This does not realistically apply to a sloped surface in a defined boundary (the boundary will be moved slightly), but for an imaginary garden it might be a start. If you must maintain a boundary in plan, you could pull the top of the shape up and then intersect the walls with a plane containing all three desired points. Remove the unwanted parts and use the new inclined plane. You'd have to redo the depression (or just pull its walls straight up to intersect with the new plane). One approach would be to draw your perimeter at the correct heights before you do anything else, whether they represent the bounds of a single plane or an undulating surface. Then fill in the surface and features. Oh and welcome to Sketchucation and Sketchup!--I hope this response isn't overkill