A Fi Series 3 Collar can connect to any Fi Series Base. Regarding connection, all Fi Collars are compatible with all Fi Bases, which means a Series 2 Fi Collar can also connect to a Series 3 Fi Base. This means you can use both bases to increase Fi Base coverage in your home. See: Fi Base: How It Works.
The Fi Series 2 Collar cannot charge on a Series 3 Base, and vice versa. See: How Do I Charge My Fi Collar?
Fi Bands are interchangeable, but limited to their series. See: How Do I Attach or Replace the Fi Collar Band?