Can being off the patch for a whole three weeks change your period week?

Are you talking about the Ortho Evra Patch?

I don't think that it will change your period week, because you get your menstual cycle every 28 days. The patch/pills/shot help regulate your period so that it's more predictable, not as heavy, and not as long. For example, instead of starting on a Monday, it can start later or even earlier, but shouldn't be off by more than a week...