It possibly may be harder to get into a rhythm when you work just the 3 days especially if you sleep "regular" hours on your days off, your body may be struggling having to switch back and forth more often than a full time overnighter would have to. I work 8 hour overnight shifts 5 nights a week. When I first started them I too hit that mid to late shift wall and it took about a month to stop hitting that wall. After that month that wall is gone and I stay awake through my shift with no energy drinks, coffee etc and another 6 to 7 hours after that. I think what helped me was doing it more nights of the week than not so it was easier and quicker to get my body used to it, I also never deviated from getting my 7 hours of sleep in right before my shift. My apartment also probably gets less light than the average home, I'm sure that's helped a little.