What is the 3-3-3 rule for working out?

There's no single definition -- and that's the first thing worth knowing before applying it. At least two different versions of the "3-3-3" circulate online: one is about structuring a routine into 3 blocks of 3 exercises each; another, popularized as recovery advice, suggests resting 3 minutes or taking 3 deep breaths between heavy sets. Neither is an established scientific rule -- they're heuristics that circulate online, useful as a starting point, not a fixed limit.

What actually matters, regardless of which version you go by: the structure of your routine (how many exercises, how much rest) should depend on your goal and level, not a generic rule from a 30-second video.

That's exactly what a routine that adjusts itself solves: you don't need to memorize rules from social media, progression is calculated from what you actually did.