no
I've been doing SS for about 12 days, and I think I'll be doing a lot of switching around of which days I work out on any given week. For instance one week might be Mon-Wed-Fri, another might be Sun-Tues-Sat.
A Sun-Tues-Sat week could never be my "constant" workout schedule, because it would require me to work out back to back days which is against the program. Is it harmful if I have some weeks like this (then start on Monday or Tuesday the next week)? Will my muscles start to shrink if I sometimes have these 3 day periods where I don't work out?
no
To borrow a phrase from Rip, you will not lose muscle if you take a three day break because nature did not see fit to exterminate the species in this way.
Your arms might fall off, though. I'm taking a 3 day break now, too. Fingers crossed I keep my arms until Monday.
depending on how you eat, you might hit some PRs that next day.. i have.
You're overthinking this. Don't worry. Train, eat, rest.