If everyone paid about 1/3 of the cost, then everyone owes 1/3 of it. Whoever keeps it should pay out what others put into it.
So, if you leave, and two others keep the router, they pay you off the $35 and it is theirs to do as they wish. If they move out, each should get their $35 share from you and you keep the router.
Ownership of the router has nothing to do with rent or any other expenses, including the Internet service. The router is an object you all bought together and own together. It is not rent either, as that would be a monthly or yearly fee they pay towards it. A flat fee payment is a "cost sharing" agreement", all 3 of you own it.
Otherwise, cut the router into 3 pieces and everyone gets 1/3 of it....