Affiliate disclosure: all three frameworks are open source so there's nothing to be paid for here. Tool links go to project sites.
i run content sites on all three. portfolio on Astro, SaaS marketing site on Next.js 15 (App Router), Mother's Day microsite on SvelteKit. here's the decision tree I use.
Short version
pure content (blog, portfolio, marketing, docs): Astro. static HTML by default, ships close to zero JS, best content authoring DX of the three.
content plus complex app behavior in the same site (auth-gated content, dynamic dashboards mixed with marketing): Next.js 15 App Router. Server Components let you compose static and interactive without picking a side.
