I doubt that there will be ww3 if US and Pak go to war. China will not interfere in a meaningful way for they are not YET capable.If there is an open confrontation between the US & Pakistan, it will result in WW3. And that is a scenario no one would want to see, including the US & Pakistan. Likely outcome is that the supply routes will be closed indefinitely, & the attacks in Afghanistan might increase. This might come as a huge blow to the US & NATO efforts in Afghanistan. There might not be overt action, but covert action.
Also routes will shut temporarily , then there will be pressure on Pak from US to open it and also the trucking industry will force the govt. So in a month or two after some work around and giving strong messages it will be bussiness as usual. Pak still needs the $$. i think thats most likely.