I suspect they are trying to do an end run around the problem of low level squishiness. They know that characters that all one shot each other is a stupid system but rather than just fix the HP calculation they have decided to just make us all start at level 3 since level 1 will be both boring tactically and also incredibly lethal. I know all kinds of people including my last two Pathfinder games started at level 2-3 because we felt that level 1 was so silly but I would really rather they fix level 1 rather than keep it and just tell us not to use it! This is the problem of trying to shoehorn crappy old mechanics into a new game; you end up doing all kinds of terrible things to support mechanics that don't work.
Even though this might 'fix' multiclassing in the sense that it will prevent multiclassing from being overpowered for melee types I think that they really need to come up with an entirely different model for multiclassing instead. Multiclassing in 3rd edition was always complete garbage or overpowered; Fighter/Mage/Sorcerer was useless and Barbarian/Ranger/Fighter/Paladin was insane. 4th edition actually had a totally reasonable multiclassing system; perfect it was not but it worked. Going back to the 3rd edition style of multiclassing where you actually take levels in other classes is simply shovelling in nonfunctional mechanics in the hopes of satisfying nostalgia. That isn't how you make a good game.