JetStream 3, a new browser benchmark, has been released in collaboration with Apple, Mozilla, and other partners. Benchmarks are crucial for detecting performance regressions and motivating browser engineers to optimize their work. This new version is built on open governance, with all major browser engines agreeing on the addition of new workloads. JetStream 3 updates existing benchmarks after a six-year gap, addressing Goodhart's Law where measures become targets. Unlike Speedometer which focuses on UI, JetStream targets computationally intensive web applications. It's designed for engine shells, enabling faster iteration for developers. Workloads represent real-world use cases rather than microbenchmarks to avoid overfitting. The benchmark prioritizes diversity across frameworks, libraries, and languages. Practical constraints include a short runtime, manageable memory and network usage, and deterministic results. A significant change is the increased focus on WebAssembly, reflecting its modern web importance. New WebAssembly workloads cover diverse toolchains like Java, Dart, Kotlin, Rust, and C#. JavaScript workloads have also been expanded with new startup and execution tests. These updates aim to improve user experience and drive better web performance.