Which three metrics make up the Core Web Vitals?

Why this is the answer

The set is deliberately small: one metric per user-visible dimension. FCP and TBT are useful but classed as supporting or lab metrics rather than Core Web Vitals. TTFB is a diagnostic that feeds LCP rather than a vital in its own right, and FID was retired. TBT never joined the set because it cannot be measured reliably in the field.