Long distance is hard and polyamory isn't easy. Combined isn't made any easier. I am not confident I could do it as a primary, let alone with a couple that are already their own primaries.
You are in quite a difficult pickle and the only thing you can do is realize the situation and learn to enjoy it. You skype every night, probably communicate often etc. You are probably ahead of the game over most ld's. Not all of them have those luxuries.
After any visit, you should now recognize the downturn. Find something in those days to occupy yourself. Hit the gym, have bike rides planned, anything to get you over that hump.
Try and setup your next visit, as soon as possible, after the previous one. This gives you something to look forward to instead of waiting around waiting for the next time you might see them.
Schedule dates. Not just regular chats, but something special. Maybe like a regular date night, on a friday or saturday and use the world of the internet to watch TV with them or something. Lots of methods for screen sharing that would bring you into their home. Being a real date might make it feel more special. Hell even just throw in a DVD you can rent at the same time etc. You get the idea haha
Long distance relationships require an inner strength and a relationship strength that isn't really matched when you have someone physically around all the time. You have to find solace in that fact, and revel in the rewards a long distance relationship will create for you. With a strong loving relationship and everything else, LD's are amazing and rewarding.
Instead of focusing on the lack of touch
I won't touch on the depression. Thats a can of worms. All of the potential ups and downs of an LD will really throw your depression in circles. I hope you can get that figured out because minus the depression can make the positives of an LD that much more bright.