I believe the question can be interpreted in two ways:
a. If Fedalovic were all born on the same date. In this case it's probable that their peaks dont coincide perfectly because of bloom factor.
b. If Fedalovic had their primes coincided together. Though it's a bit arguable.
I will try to answer both, an interesting exercise! I make the following assumptions to be objective:
a. Their form factor in each of those Slams they appeared chronologically is considered. For eg, Nadal's injury will be a factor in Slams.
b. Federer has a slight evolution disadvantage, ie, being 5 years younger Nadal and Djoker did have time to adapt to Federer, so in case I am tentative about results based on actual results today, I will give Federer advantage.
c. I will transport their careers into a totally hypothetical era. This will ignore the outside competition factors like Safin or Del Potro. Which means generally the rest of the players on tour are lesser players than Fedalovic is not capable of beating them peak for peak, and also not capable of dominating them consistently. I do this because both the generations can play this "weaker era", "tougher competition" game. As such I consider Safin owning Djokovic like theories ********.
d. There will be match-up. I cannot ignore that. Rafa has a Djoker problem, Nole has a Federer problem and Roger has a Nadal problem. That's the fact. The degree of the problem largely depends on their form. The biggest argument such a hypothetical leaves will be the draws. We do not know how the seeding will be. Is 2008 Rafa going to be ranked ahead of 2011 Nole? Who knows. But I will consider it all 3 ways. In other words I will ignore draws. In case I'm not fully sure I will give the benefit of the doubt to all 3.
Fedalovic about the same age
Nadal and Djokovic definitely were relatively early bloomers. Nadal was strong on clay, Djokovic was an all courter back then. This will naturally mean they are going to edge Federer early on. Federer will catch up. But there is a problem. At what age are they coinciding? At 20-28, Nadal-Djoker has the edge. At 22-30, Federer has the edge. I will consider 21-29 to be the most reasonable, but since they were all born mid-season for easiness sake it would be easier to consider 21.5-29.5. But neither Nadal nor Djoker have played till 29.5, so there will be too much conjecture. Hence I consider 20.5-28.5. All of them turns 20.5 on Jan 1 of some year.
I will not give a detailed explanation for sake of brevity. Just a matter of grazing through their performances w.r.t their age.
20.5: AO - Djokovic, FO - Nadal, WC - #Nadal, UO - Djokovic
21.5: AO - #Other > Djokovic, FO - Nadal, WC - Nadal, UO - #Djokovic > Federer
22.5: AO - Nadal, FO - #Other > Federer, WC - Federer, UO - Federer
23.5: AO - Djokovic, FO - #Nadal > Djokovic, WC - #Federer = Nadal = Djokovic, UO - #Federer = Nadal = Djokovic
24.5: AO - Federer, FO - Nadal, WC - #Federer, UO - #Federer
25.5: AO - #Federer, FO - Nadal, WC - Federer, UO - #Federer > Djokovic
26.5: AO - #Other > Djokovic, FO - Nadal, WC - Federer, UO - Nadal
27.5: AO - #Federer = Djokovic, FO - #Nadal, WC - #Djokovic, UO - #Federer
Tally:
Federer - 10 to 13 Slams
Nadal - 11 to 13 Slams
Djokovic - 5 to 8 Slams
Others - 3
Few points:
1. Marked with # are where I am tentative. #Djokovic means Djokovic has probably the best chance. #Djokovic > Federer means Djokovic should win but Federer is close. #Djokovic = Nadal = Federer means all 3 got equal probability.
2. Federer gains a lot of tentative votes but as I said in my assumption it's fair to do so.
3. Djokovic and Federer dont have a Career Slam, Nadal does.
4. Djokovic has slam-less years. 4 of them.
5. Federer still maintains 3 Slam per year runs - twice.
6. Nadal wins a Slam in all 8 years.
7. Federer actual tally - 17 - takes the biggest toll. Djoker's also worsens. Nadal not as much.
8. The best possible Slams would be Federer WC 2005 against Nadal WC 2010 against Djoker WC 2011 and Federer UO 2005 against Nadal UO 2010 against Djoker UO 2011. I think it's open for all 3 equally and will depend on draws. N > F > D > N basically.
9. Given Federer's superior consistency he will be mostly ranked 1. Nadal 2. Djokovic 3.
10. This is not a very meaningful analysis, for one the early age bracket penalizes Federer, and two, I had to speculate Djoker's 2015 performance.
Fedalovic about the same primes
This is better. I will consider primes as 2003-2010 for Federer, 2007-2014 for Nadal and 2008-2015 for Djokovic though 2015 didnt happen for him. I will have to be speculative there, again!
1. AO - Djokovic, FO - Nadal, WC - #Federer > Nadal, UO - Djokovic
2. AO - Federer, FO - Nadal, WC - #Nadal, UO - Federer
3. AO - #Federer = Nadal, FO - Federer, WC - Federer, UO - Federer
4. AO - Djokovic, FO - Nadal > Djokovic, WC - #Federer = Nadal = Djokovic, UO - #Federer = Nadal = Djokovic
5. AO - Federer, FO - Nadal, WC - #Federer = Nadal = Djokovic, UO - #Federer = Nadal = Djokovic
6. AO - #Djokovic > Nadal, FO - Nadal, WC - Federer, UO - #Federer > Djokovic
7. AO - Federer, FO - Nadal, WC - Federer, UO - Nadal
8. AO - Federer, FO - #Nadal, WC - #Djokovic, UO - #Djokovic
Tally:
Federer - 12 to 17 Slams
Nadal - 9 to 14 Slams
Djokovic - 6 to 10 Slams
Few points:
1. Marked with # are where I am tentative. #Djokovic means Djokovic has probably the best chance. #Djokovic > Federer means Djokovic should win but Federer is close. #Djokovic = Nadal = Federer means all 3 got equal probability.
2. Federer and Nadal's tally are affected but Djokovic's remains the same.
3. Federer has a Career Slam, Nadal arguably, but not Djokovic
4. Djokovic has 3 slam-less years at least.
5. The best possible Slams would be Federer WC 2006 against Nadal WC 2010 against Djoker WC 2011 and Federer UO 2006 against Nadal UO 2010 against Djoker UO 2011. I think it's open for all 3 equally and will depend on draws. N > F > D > N basically.
6. Given Federer's superior consistency he will be mostly ranked 1. Nadal 2. Djokovic 3.
7. When their primes coincide nobody outside Big3 are winning Slams.
8. This again has the problem of speculating Novak's 2015 performance but better than previous case.
Fun exercise!