To be fair to Chelsea; at the time each of those players where sold, the players had not reached the form that we see today (out of these players), so from that perspective one can somewhat see why they were unaware that the sales of all 3 would end up being the cock-up that it has ended up becoming.
That being said Chelsea only have themselves to blame for allowing such talented players leave the club and doing wonders for other clubs (1). Instead what they should have done with all 3 of those players (2) is do exactly same thing they did with Courtois (3), which is give them long-term loans to clubs which provide them with the right enviroment and more importantly the time to develop their potential. Thus if by chance they still ended up becoming World Class players, Chelsea would have ended up recalling those players and do wonders for them rather than watch other clubs (4).
Likewise lets also not forget the stupidity of selling Mata, Matic, Costa, Cuadrado and Filipe Luis as well, in fact had all 7 of those players not been sold in the first players then this is how the 1st/2nd teams would have ended up:
Courtious/Caballero
Azpilicueta/Zoma - Christiensen/Luiz - Rudiger/Cahill
Cuadrado/Moses - Kante/Matic - KDB/Fàbregas/Mata - Luis/Alonso
Salah/Willian - Costa/Lukaku - Hazard/Pedro
In othr words a squad which is only a GK, a Top Class CM and a few CB's away from being the best in the Premier League. Certainly this squad would be giving City/United more of a serious title challenge (and be serious Champions League contenders) rather than fighting to stay in the Top 4.
So overall yes one can blame for Mourinho for mishandling the 3 players in question while he was at the club, but the real blame for not given those 3 players the chance to develop (under their watch) and for selling those 5 other players as well ultimately has to go to the board (more specifically Tenenbaum, Granovskaia and Emenalo) and of course Abramovich. The same people who are also responsible for being completely unable to build into their title wins in 2014-2015 and 2016-2017 (unlike lets say Man United under Fergie) and for sigining players such as Bakayoko, Morata, Drinkwater, Rahman, Djilobodji, Falcao and Pato (5), players who where/are simply not good enough to play for Chelsea.
So if the club are going to get out of the mess (for a big club anyway) they yet again find themsevles in and thus be able to take on the likes of City and United in the Premier League (let alone Real, Barcelona and Bayern in the Champions League), the least Abramovich could do is sack Tenenbaum and Granovskaia (Emenalo having already left) above all else, simply because their repeated mistakes have cost the club dear...
Notes
(1) Say what you want about Lukaku, he has been reasonabally well considering the circumstances he is playing under.
(2) Because lets face it, the Chelsea first team is a pretty poor enviroment for developing young players. Simply because of the amount of pressure from the top on managers to win trophies, which in turn reduces the likelyhood of such managers gambing on unproven talent.
(3) While of course advising them to avoid having long-term relationships with other people living in the local areas of the club, to prevent the sort of siutations that Courtois has ended up in, siutations which more than anything will be the reason why he ended up leaving Chelsea for Real Madrid.
(4) Especially clubs (Man United, Man City and Liverpool in this case) which compete against you for the Title/Top 4 or who challenge you in the Champions League.
(5) Although to be fair to Falcao, the injury issues he had at the time did not help during his time at both Old Trafford and Stamford Bridge.