You are asking a question that no one can answer.
No one knows what the re-sell value of a 5DIII might be in two-three years, refurbished or otherwise. Based on recent past, DSLR bodies depreciate rapidly once a new model comes out. Will that be the case with the 5DIII? Probably, but it depends on what features the 5DIV has or if you wait until the 5D V to re-sell it.
I don't understand why anyone cares about the re-sell value of their camera bodies. Camera's aren't investments, they are tools. When it's time to replace one, replace it. If you choose to sell the old body, you'll get whatever the market it commanding at the time. If you want to increase the re-sell value at that point, send it in to Canon before you sell it and have them give it a thorough going-over and cleaning.