if you set on making a multiplayer game it would be good to at least know what you are doing, for example unity's built in internet multiplayer system has been depretiated, so you need to make your own or use a third party option.
playtesting-wise, if you have multiple screens its alway good to have a recent build of your game open so you can test stuff. sometimes even if the build and in engine game are different you can still conect and test out(though the build will not have the same functionallity as the in engine, and you would have to build to get that working)
if you want to do a local multiplayer thing thats great, though most people probably dont play gamejams submisions with their friends, so if you do add local multiplayer its probably best to do a cooperative sort of thing where you can play it 2 player but its not nessasary(that way more people can actually experiance your game)
3 days to do all this probably isnt the best idea but if your dedicated and end up doing it, im certainly interested in anything you might come up with