What is the test for apportioning damages between three separate motor vehicle accidents?

In Berns v. Campbell, supra, the plaintiff had been injured in three separate motor vehicle accidents. Three actions were commenced, and the defendants in all three actions admitted liability. The three actions were tried together and the court had to assess and apportion damages against the three defendants. Hughes J. determined that damages arising from the first accident should be assessed as of a time immediately before the second accident, with all considerations taken into account as if the second and subsequent accidents had not happened. The damages arising from the second and third accidents should be arrived at by making two assessments of global damages, one at a point immediately prior to the third accident and one after. The first assessment of global damages would be the total damages arising from the first and second accidents. The second assessment of global damages would be the total damages arising from all three accidents. The second defendant would then be liable for the first global amount less the amount assessed against the first defendant. The third defendant would be liable for the second global amount less the first global amount, and so on for each subsequent accident. (Berns v. Campbell, supra, headnote)