Cornell arrives on a season-long loan from Swansea City while Dilo and Dillon have signed one-year deals after leaving Blackburn Rovers and Hull City respectively
Swansea are happy to loan out Wales Under 21 international Cornell so that he can gain some valuable first-team experience at St Mirren.
St Mirren boss Danny Lennon was delighted to bring in the trio ahead of the new season.
"Swansea are a football club that share a very similar philosophy to ourselves so I know David will come in here and fit the criteria in every department that we want," Lennon told the club's official website
"Chris was with us for a while at the tail end of last season,"
"Looking forward he shows great potential and with the quality coaching he'll get from Paul Mathers I'm sure we'll see him challenging for the keeper's spot sooner rather than later.
"Kealan is a midfielder and we had a look at him a couple of times last season and what he showed us was great commitment on and off the pitch, and we are looking for big things from him as we look to invest in not only the immediate future but the long term future too for our football club."