Yes, most the JM Marlin marked #3's were flat top. Not sure if the change came with the rollstamp change to Marlin Firearms Co. in 1881, or a bit earlier. I have never seen a MFACo. marked #3 that wasn't concave top, nor a JM Marlin marked that wasn't flat. So I'd guess it was around 1881.
Marlin lever actions 1870's-WWI, Ballards, and single shot rifles!