The problem is the origin of your object isn’t at its center, and the Simple Deform by default will use the origin for its work (notice there’s a “Origin” slot in the Simple Deform modifier for which you can use a different point for the origin)
A simple fix is to select those 3 vertices :
Press SHIFT + S -> Cursor to Selected :
Then go to Object Mode, at the Tools tab, click on Origin -> Origin to 3D Cursor
And you’ll notice the Simple Deform will then do the work you’re wanting