The mk3 is not the problem, the problem is those heavy 2.8 zooms which the 70-200L weights about 4LBs.
That's one reason I've come to love small, light primes. 24L + 50 1.4 + 135L are fantastic to carry. If you want mobility and lens speed, try some small primes.
Not a solution for everything, unless you carry at least 2 bodies around. Again, not for family photos.
I've taken plenty of family photos with just a Minolta maxxum AF and a 50mm 1.8. Just takes a different approach.
How do you approach this type of situation, when you are 1.5ft away from your subject? No room to go back or forward: http://albums.phanfare.com/isolated/0TKGEcfb/1/5903138
Find a different approach.
Since you are a prime shooter....what approach would you take? Swap to wider prime
See, You answered your own question.
Carry multiple prime lenses in a sunny day like this for family photos...WOW, what a creative photographer NO THANKS
You yourself said it.