You have exposed a reality. No one can read the future, but it is extremelly likely that Djokovic will end up with more Majors than both Federer and Nadal. The reason is obvious: Djokovic is better than Nadal and current Federer on grass and hard courts. And 3 of 4 Slams are outside clay, which means Nadal is less favorite than Djokovic in 3 of 4 Slams.
The real race is for the number 2 spot in number of Slams. I believe (or I want to believe) that Nadal has a chance to surpass Federer's Grand Slam record by adding more Roland-Garros titles.
So, in my view, these are the most likely scenarios:
1. Djokovic with more Slams than Fedal, and Nadal with more Slams than Federer (53% chances)
2. Djokovic with more Slams than Fedal, and Federer with more Slams than Nadal (45% chances)
3. Federer with more Slams than both Djokovic and Nadal (1% chances)
4. Nadal with more Slams than both Federer and Djokovic (1% chances)