These run on a more locked-down NetBSD 6 setup. There are specific projects dedicated to modifying the Gen 5 firmware to enable SMB3. Prominent Custom Firmware Projects
Practical considerations and risks
If your Time Capsule is still working, you might wonder why you should risk "bricking" it. The standard AirPort Utility is functional but extremely limited. By moving toward a custom setup, you gain: apple time capsule custom firmware
Apple Time Capsule Custom Firmware