I don't think we need to play 3 at the back. Sanchez had one miscued header that was it. If playing 3 at the back then you need two of your centre halves to be comfortable at full back (as they will be getting pulled out wide and faced up 1 v 1 by the opponents wide forwards quite often). When we had peak Toby and Jan this was fine. Dier would drop in the make a central defender and Toby and Jan would fan right or left to cover that space as necessary. The fact that both Toby and Jan are excellent on the ball also meant that Walker and Rose could bomb on and we would be comfortable passing it out of defence with those two. As Bedford says - maybe we could recreate that somewhat by using Davies as the left sided centre half, on the right side though we either have to have Toby (who probably no longer has the pace) or Sanchez who doesn't have the ability on the ball. In that game against Man Utd I think having a proper defensive midfielder who was able to successfully stop Fernandes dictating the attacking play for Man Utd would've been more effective than a third centre half. Having one very effective DM would also have freed up one of our deep two to get a bit further forward and actually give us an option to be able to play our way out instead of having to continually go long and turn the ball straight back over to Man Utd all the time.
If we can get a proper defensive midfielder in next season who is also a good passer of the ball, then I think Sanchez and Dier would make a good complimentary pair at the centre of defence. I would then just play Sessegnon and Aurier as attacking full backs, akin to the way that Walker and Rose played under Poch in the seasons that we finished 3rd and 2nd, if we can afford to get in better full backs then happy days, but if not lets play to both of those players strengths.