The two wide CBs need to cover the overlap and Souttar and Collins are quicker and more reliable. Baath would terrify me in either of those berths so he's got no real option in a 3.
Chester would be even worse and he doesn't seem to trust Lindsay. Which is a shame because I think Collins, Souttar and Lindsay would be worth a try.
That is the 3 he should have picked today if he’s got to go with this crap.
Lindsay must be showing nothing in training because I think left in a three is probably his best position. His distribution is pretty good and with 2 other CBs alongside him any defensive vulnerability would be covered off.