Each week players receive bonus Legendary Marks from the Weekly Heroic Playlist. These awards are received at the Account level rather than the character level - as was the case with Year One Weekly Heroic Rewards.
Also the 3 run "limit" seems to just refer to the weekly Mark BONUS
Meaning subsequent runs after 3 give out less marks?
Not sure on this since the wording isnt clear
Luke Smith just tweeted a few things about legendary marks if you're interested.
"Legendary Marks II: But, our goal is to have players finding Rare/Legendary gear in the world or as rewards, rather than buying from Tower."
Top Page: here, have a Hand Cannon: