Which one will be a better option chosing iit bombay with mech. or roorke( or karagpur ) with cs.??

It depends on which you are more into. CS got huge placements everywhere. No doubt about. All the three got good infrasturcture but when in comparision IITB has top placement whether it is average compensation or infrastructure. But always give least preference to IITR and then to IITG. I’m giving you a rough idea, choose which one suits you. All the best