Stevenson's fresh deal will run until at least the summer of 2016, while McComb, who signed professional terms with the club in 2010, will remain until June 2015.
Doyle, 23, will stay on until the end of the 2013-14 season, with the short contract reflected by his recent injury troubles.
"I am delighted that these three players have committed to Ulster in the long-term. Lewis and Neil have been important to us all season, while Sean showed in his first few games what an exciting prospect he is," Humphreys told reporters.
"I have no doubt that they will only get better as players and that is good for Ulster Rugby."