The short answer
Transport Fever 3 does not include Steam Workshop integration. Its official in-game mod browser is realized with mod.io and is designed to serve players across PC and console platforms.
Why one mod destination matters
Steam Workshop is tied to Steam. A separate Steam-only library would split discovery, updates and availability from players on Epic Games Store, GOG, PlayStation and Xbox. The official FAQ describes mod.io as the single destination for the cross-platform mod ecosystem.
What players should use
Use the in-game browser for supported discovery, installation and management. Check version compatibility and dependencies before enabling content. Manual download and sharing remain possible according to the FAQ, but require careful source verification and file management.
Save compatibility still matters
Cross-platform distribution does not guarantee that every combination of mods is compatible. Back up saves, add content gradually and test after updates. Mods that add vehicles, stations, industries or scripts may leave dependencies when removed.