I would like a settled system and don't think 3 at the back suits us at all, but I do sympathize (a bit) with TM with regards formation changes as I think he is facing the challenges below:
-Finding a spot for the returning Lenihan in the 11 (one of the best performers last season).
-Trying to get Armstrong playing more centrally without dropping Graham who's scoring regularly.
-Introducing Amari Bell into the 11 without dropping Williams, who has his failings but still seems to merit a place in TM's eyes.
-Having to accommodate our best player in Dack who can't play in a 2 in midfield.
With that said, in my view is he needs to be a bit more ruthless and have confidence in this squads ability to be beat anybody in this league, without overcomplicating the selection to compensate for the opposition's strengths.
Forget the back 3, it hasn't worked and whilst Bell seems to have the attributes, on paper, to be a decent wing back it clearly doesn't work on grass. IMO he'd be more effective attacking from deep, overlapping an advanced left sided attacker. Neither Nyambe or Bennett are capable of playing right wing back either. He was delighted with Mulgrew and Lenihan in as the central defensive pairing last season (evidenced here > https://www.rovers.co.uk/news/2017/may/they-give-us-a-solid-base-to-work-from/) ... so why not just go with them as the pairing again now? Downing has proved capable but he was only ever signed as Lenihan cover and Williams doesn't merit a start at CB.
My team, therefore would be:
Nyambe Lenihan Mulgrew Bell
Payne Dack Armstong
Antonsson off the bench for Graham and move Armstrong up top for a different dynamic should the game need it.
I'll go for a 2-1 win on Saturday. Armstrong and Dack to score.