assign this (keybind) macro on your toons

/script AcceptGroup();
/script AcceptQuest();
/script AcceptTrade();
/script RetrieveCorpse();
/script RepopMe();
/script StaticPopup_Hide("PARTY_INVITE")

you will accept a quest on your main, then open your list of quests and share the quest with your party, then click the keybound macro that you have assigned for each member of your party (in your case, the other two party members)