That's not true, sometimes people think you'll get by with google.
Depends on what you shoot indoors, the 1d4 is really king with the iso, and even for outdoors, the 1d4 would work well with the 70-300 and 100-400.
The thing is, the 1d3 has issues with focus, even with a good copy, they hesitate to focus sometimes.
This is what I would do sell the 50d and the 70-300 if you arent beforehand, and grad some primes
-canon 85mm 1.8: awesome focus
-canon 135 f2
These will work for volleyball, basketball, etc.
On a baseball, football, soccer, lacrosse field('s), etc. the 100-400 will work great, even at night, because of the large lights, and the great iOS of the 1d4.
-one last thing, you could always sell the 100-400 for a 70-200 2.8, get the 2x converter, and a monopod, and then your set.