Often? I agree that liquidation preferences are often overlooked by startup employees but I wouldn't say 3x is common or generally acceptable.

The National Venture Capital Association (NVCA) benchmarks[1] show that over 95% of term sheets across all funding rounds include a 1x liquidation preference.

[1]https://nvca.org/model-legal-documents/ (it's in the Enhanced Model Term Sheet v2.0)