I think the tasks were specifically designed for the trio. Harry was good at flying, Hermoine at logic and Ron at chess. My question is how did Quirrel get through? Putting Fluffy to sleep should have been no problem, since he already knew how. The roots should have been no problem, and he may have been able to figure out the potion logic. He could have used Accio on the key, but how did he get past chess? Would LV, as a passenger, have trusted his logic? I think not.