You don't have to watch them in order, when I was done with S2 I skipped ahead to San Junipero and then went back to Nosedive. Barely made a dent in my experience, I'd figure.

The first three episodes that form S1 are a fantastic introduction to the series. They dragged an intense tableau of emotions that really exemplified the BM experience for me.

S2 is a lot more of the same level of quality and tone, with only one incongruous addition. (That would be The Waldo Moment [S2E3], which is hit or miss with most people.)

Even though their storylines are unrelated, White Bear [S2E2] and White Christmas [S2E4] are part of the same "moral canon" and I consider White Christmas to be a sequel of sorts to the former. I highly recommend watching them within a few episodes of each other, if possible.

S1 and S2 feel like "Classic Black Mirror" to me.

Meanwhile, S3 is "Netflix Black Mirror". It gets funkier and more eclectic with its storytelling.

Basically, know that you can go in any order you like, but the first two seasons also function as a cohesive set in comparison to the Netflix ones, which are a different beast, for the most part.