Great tips! I've been practicing your 3x3 rule for about 30 years, learning about any subject, never had a name for it before.
I find the 3x3 (or however many is appropriate/available in your situation) rule really useful for 2 reasons:
because while each resource covers the basics/the facts, the hidden value for each is the discussion around each topic of why something is useful, tips and tricks to learn or watch out for, etc. Each resource will have different sets of tips, different anecdotes on why something is a timesaver, different perspectives, etc.
because just like every learner has a unique learning style, every instructor has a different teaching style, and reading/hearing different instructors cover a topic contributes to a richer, fuller understanding in the end.