Martial was incredibly productive in the first few months of the season, and forced himself in the starting 11. Him and Rashford were rotating starts for the first 2-3 months basically as they were both in very good form, while Lingard hardly featured. Martial eventually made the spot his own and was starting pretty much every meaningful game with the rare rest (but still getting subbed on) in less meaningful games between the start of November and the end of January. Problem is he then lost form and since his goal in the 1-0 Burnley win, he hasn't scored and has had just the 1 assist (vs Chelsea, a game where his second half 15 minutes were really bad so got subbed). He lost form, so lost his spot. Standard.
Rashford in that prolonged spell that Martial was starting basically every game ended up losing his form and barely featured until about a month ago. Start of the season he was very good, lost form, lost his spot, then had bad sub appearances then he got a bigger rest. Normal behavior with a young player.
Lingard had just 5 starts before the Watford game on November 28th. 2 in the league cup, 1 in the league, 2 in the CL (one where he got subbed off at half, the 2nd being the last game which was a dead rubber). He barely played, but then had a few brilliant performances vs Watford and Arsenal and transformed as a player. Worked his way into the side, and played consistently well. Lost a bit of form a couple of times but then had big impacts from the bench (like vs Chelsea) to get right back into the 11. So again, pretty standard in terms of how Mourinho dealt with him.
It's not a black and white situation like "this player has a better goal/assist per 90m ratio then the other one, therefore he should start more. It's a long season, things change. I'm not a big fan of Mourinho but he's dealt with Rashford, Martial and Lingard pretty normally. The only one who can feel hard done by IMO is Mata probably because he always gets dropped in big games, and even then he still plays loads and had recent big game starts vs Sevilla and Liverpool.