The most important thing that I would recommend is not leaning over sefarim for three sedarim. Sitting is the new smoking, as they say. Why not try standing up and using a shtender every so often? Or using a small, portable table-top shtender so that you don't need to lean forwards? Changing up your posture and position every so often will reduce strain on your back and neck, and is generally a healthier option than physical inactivity anyway.
As a bedieved solution, I would recommend seeing a good physiotherapist who can provide you with exercises that you might be able to do every evening before bed, every morning on awakening, or at a time of the day that suits your learning schedule. But as they say: prevention is always preferable to a cure!