We had a similar problem, so one day when a technician came out about a different issue, I asked him about it. He explained that the southern CA sun was heating up the roof, which then heated up the styrofoam insulation that the air ducts are cut into. Until the AC's could cool down the styrofoam, we wouldn't get much cold air out of the vents.
That's when I started looking for something to put on the roof. We had a neighbor that had one of
these. I ended up getting one and it made a HUGE difference!!!
With all 3 AC's running, I could keep the inside temperature 25-30 degrees cooler than it was outside when it got over 100. Less than 100 outside, the bedroom AC would cycle off and on to keep it around 72. The AC in the middle would run most of the day to keep it less than 74, with the rear AC helping out some too.