yt3

yt3 is a hostname used by YouTube’s content delivery network to serve certain static media assets, such as video thumbnails and channel profile pictures. It is not a standalone service or public API. The label commonly appears as part of image URLs on YouTube pages, for example in links that resolve to image files hosted on domains such as yt3.ggpht.com or yt3.googleusercontent.com. These hosts are operated by Google and are used to host publicly accessible media associated with YouTube channels, videos, and other assets.

The exact allocation of assets to the yt3 host can vary over time as Google manages its CDN infrastructure; the "yt3" portion effectively acts as a host selector within a larger domain. Because the URLs point to Google's infrastructure, they are typically served with standard HTTPS, and the images are accessible to any user who can view YouTube content.

From a developer's perspective, yt3 is not an API or a service you can query; if you need data about YouTube channels or videos, you would use the YouTube Data API. The presence of yt3 in a URL is incidental to the asset's hosting domain rather than a user-facing feature.

See also: YouTube, Google Content Delivery Network, ggpht.com, YouTube Data API.