I don't watch Portuguese football enough to give an educated opinion on this however I don't think that ETH wants a 3 CB defense. I very much doubt that anyone wants to touch Shaw at this point in time either. He's been injured for far too time at this point.
Many underestimate how important someone whose good in air is in the EPL. Liverpool has absolute beasts in Van Dijk (3.8) in aerial duel won, Konate (3.1) and Quansah (2,3). Arsenal has Saliba (6ft4), White (6ft1), Tomiyasu (6ft2) and Magalhães (6ft3). City aren't as imposing but most of their defenders are 6ft+ tall. The converted CB is not a novel thing either. ETH did it at Ajax, Arsenal look set to do it with Timber (Ben White can play as CB as well) etc.
I believe that ETH had ignored that aspect at first. Martinez (5ft9) and Malacia (5ft7) were introduced in a high line defense that poorly suited the only beast we have in aerial duels (ie Maguire). That placed too much pressure on Varane, Shaw and Dalot with only Varane being 6ft3. With Shaw gone due to injury more pressure ETH had to choose between compromising the high line thus reintroducing Maguire or play McT to help out. Either option was a poor solution to the problem.
In my opinion this issue is now being addressed. De Ligt is 6ft2 and is better in aerial duels then Varane is. Casemiro is set to be replaced by another 6ft player in Ugarte. If we add Jarrad then we'll getting a 6ft4 CB whose better n aerial prowess then the likes of De Ligt, Varane and Stones let alone the likes of Martinez. Hence why I can see us returning for Jarrad. He offers a certain mobility and ability in aerial duels none of our current defenders can provide. He would also end our reliance on McT as well.