People are way too hung on three at the back. Amorim's basic approach is no different to most big teams these days - a combination of pressing and possession, the aim to play the game in the oppostion third, width on both flanks, a couple of players moving in and around the half spaces outside the penalty area. Its a common template. The difference between his style of football and many other coaches who play the same way but with 4 at the back is smaller than the difference between counter attacking football and possession football.
The main concession to his style is needing either attacking full backs, or wingers who can do a shift defensively. This is different from those managers who prefer inverted full backs or inverted wingers. But really, that's not a huge issue, plenty of managers use attacking full backs. Beyond that, the only other issue is the potential to have an extra first team quality CB, which is not really an issue if you buy decent players in the first place, just sell one on.
You almost never get a new manager coming in who doesn't want to make any changes to a squad. There's always some turnover as new managers do their own thing. What matters is that you can adjust a squad to the new manager's style with 2 or 3 new players, rather than needing 7 or 8.