First make sure they are truly abandoned. You need to watch the nest from a distance for at least 1 hour continually, to be sure no parent is coming to the nest. If only one parent appears to be tending the young, it may be able to raise them unassisted. If the male disappears, the female will be able to raise the young, for she will brood them at night. The male bluebird, though, lacks this instinct, and though he will feed the young, he won't brood them overnight. He'll be able to keep them alive only if they are more than a week old, and the weather is warm. If the chicks are warm to the touch and seem well fed, leave well enough alone.
Abandoned young may be weak and cold but can survive for 24 hours. Keep them warm and in a dark box and take them immediately to a licensed bird rehabilitator. It's illegal, not to mention very difficult, to raise young birds, which need food every 20 minutes when small. They have a much better chance to survive with someone who has experience. To find a licensed bird rehabilitator, call the Project Wildlife San Diego County Emergency Wildlife Hotline at 619-225-9453.