There are some great portable nebs out there. $80 to $190 and most insurance will cover a portable for a 3yo who can't use an inhaler. Personally, I would get one with disposable batteries and I've seen one for about $85, but I can never remember the names. They easily fit in a purse.
You don't need an ultrasonic. They are hundreds of dollars more.
Check with your local respiratory equipment supply companies like Apria or Lincare.