It is only a rule (not a requirement for Sacramental validity - you only need one valid Bishop). It has, indeed, been around for a long time - the rule was established by the first Ecumenical Council, the Council of Nicea (which also gave us the Nicene Creed that we recite at Mass).
In 313 AD, the Edict of Milan ended religious persecution in the Roman Empire, and the Roman emperor (Constantine) converted to Christianity. Suddenly, the Catholic Church - which has been illegal and persecuted for centuries - was in with the in-crowd.
This also meant that being a Catholic Bishop - previously an invitation to torture and martyrdom - became a path to wealth and prestige. Con-men fraudulently presented themselves as Bishops in many provinces. It was difficult to verify the validity of their “orders.”
This was an early crisis for the Church. Only a valid Bishop can ordain someone Bishop. If these false “bishops” performed “ordinations,” they would be ineffective. The new “bishop” would not really be a Bishop (though he might not even realize it). This crisis could bring down the whole Catholic Church. Without valid Apostolic Orders, the Church cannot exist.
Twelve years later, at the first Ecumenical Council in 325 AD, the Church addressed this problem with the three-consecrator rule. The Church took on a sacramental crisis with pure mathematics - and it was absolutely brilliant.
Suppose that, for sake of discussion, we stipulate that one in ten early Bishops was not really a Bishop. If we have a rule that requires (at least) three consecrators, what are the odds that all three will be invalid bishops?
We only need one valid Bishop. If the other two are invalid, they cannot pass on their invalidity.
The odds are 1/10 * 1/10 * 1/10 = 1 in 1000. There is a 1 in 1000 chance of selecting three invalid bishops at random from a pool of bishops in which 10% are invalid. Only one out of every thousand Episcopal consecrations would be invalid.
The Nicene-era Church surely did not even have 1000 Bishops - so it is statistically certain that every post-Nicene consecration would be valid.
The math works even if we stipulate that HALF of the Bishops were invalid. The odds of selecting three invalid bishops from this pool is 1/2 * 1/2 * 1/2 = 1/8. One in eight consecrations would be invalid in the next generation - better (we now have only 1/8 as many invalid bishops as we had before) but not enough to statistically insure validity.
But we get pretty close to assured validity in the next generation. We now have a pool of Bishops which is 1/8 invalid. The odds of selecting three invalid bishops from this pool is 1/8 * 1/8 * 1/8 = 1 in 512. I doubt the Nicene-era Church had 512 Bishops. But it surely did not have 512 * 3 Bishops (ie, 1536 Bishops). There is a 1:512 chance of getting ONE invalid bishop - but ONE invalid bishop cannot pass on his invalidity under the three-consecrator rule. We need THREE invalid bishops, so we have to beat those 1:512 odds THREE TIMES. The odds of that are 1 in 1536.
The three-consecrator rule is brilliant. The line of valid Apostolic Succession actually purifies itself over time. It is absolutely inconceivable that we could have even ONE invalid bishop today.
When the Church claims that Her line of Bishops is absolutely valid, you can take that to the bank - thanks to the First Council of Nicea.
Them Council Fathers were some smart dudes.