I’ve just completed the first two missions of Halo 3 on Legendary and the “Change of Scenery” challenge still says I have completed zero missions from Halo 3 or 4. Other missions which should have updated (Battle Hardened and Narrative Navigator) also did not update.
Battle Hardened and Narrative Navigator both updated properly after completing a mission in Halo 1 on Legendary, so this problem may be isolated to Halo 3.