The Lions, who will play at The Willows next season, have recruited impressively this close season, having already brought in vastly experienced former Super League duo Mark Smith and Tommy Gallagher.
Ainscough joins namesake Martin in the squad after appearing in a spell on loan from Leigh last season, while prop forward Morrison also joins from Leigh, having been released following 48 appearances for the Centurions.
Ashall is a product of the St Helens academy set-up, and can play at either half-back or hooker.
Director of Rugby Paul Kidd said: “All three of these signings will be assets for the Club. They will be competitive and there is now competition throughout the squad.”