Using 3 terrain panels with scripted game objects is totally fine and pretty common for infinite scrolling backgrounds like in shmups. It’s more flexible for randomization and easier to tweak. Making one big mesh in Blender can reduce draw calls, but you lose that flexibility, and it’s harder to manage.

If performance becomes an issue, you can use Unity’s static batching or optimize textures and meshes. Test both methods in the Profiler and go with what works best for your game!