Is the Apple Watch series 3 still waterproof if the screen is cracked?

Any cracks to the screen can allow water to seep in. I would have it repaired or avoid getting it wet.

Nope, it's not waterproof anymore.

I had the same problem, same corner, without a major hit or drop. Took it in to Apple. If it's been more than 60 days since purchase and you didn't get AppleCare you may have to pay $289 for repair/replacement, unless this is a material/manufacturing problem.