Three.js is the more common of the two in our index: it runs on 331 websites against 17 for PIXIjs, and 1 sites use both. Counts come from real scans, not vendor claims.
PIXIjs is a free open-source 2D engine used to make animated websites and HTML5 games.
Three.js is a cross-browser JavaScript library and application programming interface used to create and display animated 3D computer graphics in a web browser using WebGL.
1 site runs both PIXIjs and Three.js. That is 6% of the sites using PIXIjs, the smaller of the two.
Want to know which one a particular site uses? Look it up and see its whole stack, or compare two sites side by side.