Sorry for bad english
haveing online co op in a lego game said to be but a dreami'm hear to help ya learn hwo to do it not just with lego batman 3 but any lego game and any local co op game the first step is downloading https://parsecgaming.com/ what parsec is its atollthat turns any local co op game in to online mulitplayer some games will require p2 to have a controller like cuphead but for lego games it doesnt matter if p2 has keybaord or controller install it when it asks you u want controller support u click yes u want that then u gonna get another pop up asking u to installsomething else u wanna install this to so it works right after u done installing make your account.

so now that ur all set up have the friend u want to play with do the same make there account and install it
now add your laptop or desktop or what ever your pc name is to the list besure to enable hosting tell ya friend to do so also
then create a party set up the party besure to have said game running in back round when you start makeing the party
set every thing up then add the game to the aprooved apps list then create party then ur party should be on the left side
then you should see a button that says copy or invte copy the link send it to ya friend and wait for him to join the party then tell ya friend to click play then u go back in to your game make sure your friend doesnt touch nothign till ur in game level and rdy for him to press F2 or start on said controller or keybaord after he does that boom you are now playing online with ya best friend.

playing with random ppl u gotta becarefull not to add anything other then the game to the list as they can see what you see they can see your steam overlay and type on ya frinds message box and click like its theres best to stick with ya best friend also both players dont need to have the game only 1 person needs to have the game for to play together so have fun i hope this helps