Well, it all depends on the map. In some cases one very specialised shield city using 2 food cities to fuel it may out-produce 3 balanced cities but that would be rare, in most cases balanced cities will have summed up production higher.
Well, I've already explained why a single science city will quite often be the best choice over multiple science cities.
As for production, well it's more compicated which is why I didn't specify a single production city originally. For a start using the example of 3 cities is unfair since you'll really only need the extra food from one decent city to use most of the production city's production. Take two cities - one production city and one source [of food] city. The production city contains the Ironworks. Assume both cities contain the same production buildings. Assume that there is no happiness cap (for the same reason that was assumed about the science city).
1) If the production city has an unused 3 production - an unworked grassland hill mine - then it will make more production out of the 2 food than the source city unless the source city has an unused tile worth 6 production or if both cities contain a forge, a factory and power and there is an unused tile worth 5 production.
2) If the production city has an unused 2 production - an available engineer specialist or Angkor Wat-enhanced priest specialist - then it will make more production out of the 2 food than the source city unless the source city has an unused tile worth 4 production.
3) If the production city has an unused 1 production - guaranteed since unused citizens provide 1 production - then it will make more production out of the 2 food than the source city unless the source city has an unused tile worth 3 production or if both cities contain a forge, a factory and power and there is an unused tile worth 2 production.
But it's not that simple. Unlike gold and science, production isn't summed up over your entire civ. So while the source city might be able to make more production out of the 2 food, if it doesn't already have significant production then the extra production it makes won't actually be used for much. Simply put,
increasing the production city's base production by one is often more valuable than increasing the source city's base production by two or even three.
What if the Ironworks hasn't been built yet? Well, the main reason to create a production city in the first place is to have a city that produces lots of military units. So the above is largely the same if the production city contains the Heroic Epic. I think it would probably be wasteful to put the Heroic Epic and the Ironworks into the same city under the above game mechanics since it would probably be better spent in a second production city - 2 production cities and two source cities.
a) a bit of realism, current situation is so unrealistic it breaks the game immersion
There's enough unrealistic aspects in city management that I find it difficult to believe that the lack of food transfer is the element that breaks the game immersion.