how to run 3 job(a,b,c) parallel in unix shells script and after will complete d will start and we have to handle the error also
Please don't overwhelm us with so many details!
Just off the top of my head ... as a general approach ... something like
while (some condition checking that all three procs are still running ... maybe a ps -ef |grep )
But, we'd really need to know what it is you are really trying to accomplish, instead of your pre-conceived solution.