Helping others spend their money...that's what we do here at the TDP forums

I like your plan, with one exception. If you are indoors, the 100-400 is probably not the lens that you want. I believe Neuro recommended the 200 f/2 L IS. Depending on which event you are going too and how close you are to the action, the 300 f/2.8, 135 f/2 L or even the 85 f/1.8 may be good options. It would be something to play around with, how close are you to the action, and then find the right lens.

I haven't yet played with the 5D3. Maybe, if the 5D3 is as good at ISO 6400 or 12,800 as some have said, maybe you could use the 100-400L indoors. But that is something I'd want to test out first. Since f/5.6 lets the same amount of light hit the sensor all every focal length, you could use your current gear, set it at f/5.6 and f/8 and go to a stadium or similarly lit indoor area, and test out if you need more light or if you would be satisfied with the results.