If you're doing an upper/lower spilt, 3 days in a row would be fine. If you're not feeling up to it, though, skip it. Personally, I wouldn't worry about doing a full body or "catching up". If you have to skip, just let it go and keep on your regular program. I'm sick this week, and Monday I did my upper body, but left out over head press becuase I didn't think I could handle it. I just walked slowly on my treadmill for my cardio. I didn't work out at all yesterday, I just felt too crappy. (Lower body). Today is my strictly cardio day. I was going to do another easy stroll on the treadmill, but whenI got up, I felt woozy, so I let it go. I hate missing workouts, but sometimes it can't be avoided. If you know ahead of time you're going to miss, you could rearrange your schedule, but otherwise, I'd just let it go, and stick to the regular routine. That's what I plan to do. I'm doing upper body tomorrow, since it's my normal thing (if I feel well enough), even though I haven't done lower body yet this week, that will be for Friday. The only thing I'm "making up" is the booty and abs program I'm semi-following. I'm going back to the workouts I missed instead of skipping them. They're 30 day programs, this will just extend that a bit, extra rest days, I guess!