Modric doesn't have the same function as Xavi and Scholes as far as I'm concerned.
Xavi and Scholes (Post 2006) were both organisers. Modric is a needle player like Iniesta.
Xavi was the most complete passer at all ranges of the 3. His technique was excellent and he almost always passed it to your stronger foot and in a way that allowed you to take half a touch and control it easier. First touch wise he has the best first touch I have ever seen and almost always never lost the ball.
In terms of long Passing I would give it to Scholes but Xavi was his equal, short passing there is no contest between the three, Xavi is the best short passer for the reasons I gave in the last paragraph. Throughballs Xavi is the best followed by Scholes and Modric. Modric is relatively weak in this area. Xavi has pssibly the best slide rule pass, it is up there with the best I have seen such as Michael Laudrup and Messi.
In terms of ground coverage there is no contest. From 2008 to 2014, Xavi covered the most ground in every game, made the most passes every game and was involved in all 3 phases of his team, especially in the 08/09 season where Yaya Toure was the holding midfielder and Xavi had to drop deep to collect the ball from the defence, organise the game in the midfield and then make through balls in the final 3rd. That is the most dominant season from a midfielder in the modern era.
In terms of press resistance (facing your own goal) Xavi is arguably the most press reisistant central midfielder I have ever seen. Modric is his equal in this area and Scholes is the weakest, particulalry when he transitioned from his mobile days to the conservative deep lying playmaker post 2006.
In terms of Ball Carrying (going towards the opposition goal) Modric is arguably the best I have ever seen and is the great needle player of this generation alongside Iniesta. Xavi is a bit behind in this area and Scholes is far behind. Modric will use his press resistance and the his ball carrying to relieve his team of pressure which is why he is one of the best players in that position.
In terms of organising/dictating the game, Xavi is the best I have ever seen. He is the main reason it was very difficult to counter attack Barcelona and Spain as he got the ball in congested areas and did the pelopina/faints to avoid getting into culdesacs and that allowed his team to stay very high up the field and not reset attacks when he was pressed. He was the apex of that. When Xavi's physical attributes declined (he had achillies problems for a long time due to covering the most ground in every game for 5+ years), Barcelona and Spain where much easier to counter attack. We saw this especially with Spain as he did not have the legs to carry the ball in all areas and 'out fires high up the pitch' as he could not be all over the pitch so they got dismantled with counters through the centre that would not have happened in Xavi's hey day. The next is Scholes and then followed by Modric.
In terms of vision to spot passes, Xavi again is the best for me followed by Scholes and then Modric. Modric has a lot of situations in Madrid where he could play between the lines but then he checks back and then plays an outside the foot pass to the opposite flank missing a runner. I have seen this in almost every game in Madrid.
Another area is using your weaker foot. In this area Scholes is ahead of both Modric and Xavi as Xavi had to use feints when he couldn't use his left foot and Modric resorts to using the outside of his right foot to try and create the angles if not he checks back using a feint like Xavi. Scholes long range passing with is two feet was superior.
Defensively, when the team sits in a deep block, Modric is better in one vs one situations whilst Xavi was better at covering passing lanes and intercepting when pressing. I don't think I need to talk about Paul Scholes in this area...
The above cannot be analysed without the context of the teams they will play in and the opposition. If I had to pick one of the three (ideally I would partner Modric with one of them), I would follow a general rule:
In a team that depends on tranistions and there is no controller, I would take Paul Scholes as he is a good passer with both feet.
In a team that depends on transitions and has a controller, Modric as he a fantastic ball carrier.
In a team that has a needle player (Modric/Iniesta type player) I would pick Xavi as he would organise the game and get the needle player in better positions to ball carry in the final 3rd.
In a team that plays high up the field and faces low blocks regualry, Xavi as he is extremely press resistant coupled with the ability for his team to not reset ball possession (passing back to the goalkeeper).
It has to be said that Paul Scholes was unfortunate to not play with a needle player in his career. Modric played with Xabi Alonso and now Kroos (Controllers) Whilst Xavi had Iniesta.
high five then!