So, I would Draw the triangle in plan, go to Modify>Convert>Convert to Subdivision. Start buy setting the subdivision iteration to a low number, like 0 in the ensuing dialog. Change to a front or side view.
Select the edit subdivision tool in the 3D modeling toolset, first mode or operation (transform), select one corner. Hover over the dragger until eh Z axis turns yellow and drag up. Tab into the floating data bar and set a height. Remember, the triangle is the ground, so set the heights above the bottom for the heights from the low point of the swoop. Then I would raise the the thing. Alternately, you can raise the flat surface first.
Changing views helps, working in OpenGL may help.
Select the Split mode of the Edit Subdivision tool and select an edge. drag towards the center. Do the same with another edge. Use the transform mode to select the new center point. use the dragger to have the surface bow down in the middle. By default, the dragger is constrained to XYZ movement, although you can rotate those axis. For what you want, I don't think you want or need to do that.
It's tricky at first, but once you get it, it all makes sense.