Often in these cases rust is kind like an iceberg with the outer skin being the water level. The deeper into the metal you get the larger the rust area like how an iceberg gets bigger under water. Your plan sounds good but I would not bother with the 3rd external plate. If it's welded properly it will be stronger than original with the inner and flat layer and will look nicer. If the last layer is beveled so you can get a layer of weld deep and then another layer on top of that you can ground the weld flat and make it look good. If you are concerned that the weld will weaken the frame then you can do a fish plate to spread out the weld area. These are boxed frames so they are quite strong unlike what you'd have with similar rust on C-channel. My 2 cents.