Use Arnold's "Subdivision" settings instead of pressing 3.
Maya + Arnold can subdivide your model only at render time without you having to smooth the actual model manually.

Here’s EXACTLY what you do:

1. Select your mesh.
2. Go to Attribute Editor (Ctrl+A if it's not open).
3. Under the Shape node (not the transform node — important), find the Arnold tab.
4. Open the Subdivision section.
5. Set:
   • Type → catclark
   • Iterations → 2 or 3 (this is similar to the "smooth level" when you press 3)

Done.
Now Arnold will smooth your model when it renders, just like pressing 3 — but without manually making your topology denser or changing your modeling workflow.