Besides diminishing returns, the main reason you keep them the same is because how the hit table works. I forget what order this is in but it works like this. Game rolls dice for block (if 20% you need a 20) next game rolls for dodge(if 15% you need a 15) game rolls for parry( you get the idea). So its best to try and keep them even for best avoidance chances. 33/33/33 would be better than 50/25/25.