Have seen several statements that Colin has been confirmed for the third season. Can’t find the source of the confirmation. However, since it is only 5 episodes, it seems likely he will return for a portion since his part could probably be filmed in a matter of days/weeks. (He appears to effortlessly, fluently fit short projects between and among the larger ones. Clever man!)
It is questionable any intense look at DS Anderson’s past will happen, though there may be a suggestion of it. This season is meant to wrap up the story between Stella and Paul.