The Ireland internationals have penned new deals to keep them at the Kingspan Stadium until the summer of 2021.
All three players have become key men for the province, with Marshall in particular starring as he's managed 21 tries in 111 appearances, while Herring and Treadwell have recently been recalled to the international set-up.
Director of Rugby Les Kiss said on the club's website: "Luke has been one of our most consistent players in recent years; he's a robust presence in our midfield and is a brilliant link man in both attack and defence.
"Rob and Kieran have also been in excellent form this season and I was delighted to see them earn recalls into the national squad for this month's fixtures.
"Rob has really grown as a player since his arrival at Ulster five years ago and he is widely regarded as a highly skilled hooker who leads from the front.
"Kieran has made a big impression over the past 18 months and it's evident that he possesses all the attributes required to become a top class second row; he's a big, mobile unit and is a good student of the game.
"All three players are determined to continue to work hard to improve and that is exactly the type of culture that we're trying to build here."
He added: "With increasing player movement, it's vital that we continue to retain our best talent, and to have Luke, Rob and Kieran commit to long-term deals is a huge boost for us. They will play central roles in our programme over the next few years."