Are the countries hosting the world cup not supposed to individually develop their stadium on their own? Since it's not like the three countries involved in hosting the game will come together to make one stadium available for the game.
Even if fifa is going to allocate monies to this countries for stadium development, it will be on individual scale, country with an already developed and well furnished stadium will get less allocation for minor fixes, while country with a stadium that need a total rehabilitation will get a higher allocation to make sure all is well put on place before the start of the tournament.
In my own opinion, The three countries should get equal share of the FIFA's funds regardless, except if they come in agreement to that to take cut. There are a lot to be done when it comes to hosting a world cup in a country, which is not only on stadium development but other side of sporting areas that will play part in contributing to the successful running of that world.
If larger share is given to such countries that needs total rehabilitation, then this way it could encourage more politics to come into the football governing body and potential world cup hosting countries would be in reluctant to develop their stadium hence they know they gonna get bigger funds from FIFA and may go to lobby to become world cup host countries in the future in order to get that lager share of FIFFA funds to develop their stadium.