For the most part, the Big Three manage to compromise on what constitutes "Mage territory" or "Uratha territory". The Kindred, of course, have no use for any territory during the day, and even when they do wake up, neither the Mages nor Uratha want to compete with them for feeding rights.
So in some cases, the supernaturals can agree to share a territory. A good example is the British Museum. This is a Kindred Elysium, and security (at least security against London's shadow community), is maintained by House Skade, a Kindred group. But by long-standing tradition, Skade maintain the sanctity of the Museum on behalf of all three communities, and all three share access to the Museum equally.
The Temple Church has been recognized as unclaimed neutral ground for centuries; the Temple Underground station (fairly) recently gained the same status. Other areas have a particular significance for particular groups.