Re: You know all those things that "baby trainers" acuse babies of...
Aw, stringbean, it sounds like normal separation anxiety at his age.
OP - when children learn the power of communication, they learn that specific requests get them specific things. They also can't describe complex things well. They might say they want a grilled cheese, but not be able to tell you they're thinking of a grilled cheese like the one they had last week cut into triangles with a certain bread. So they become emotionally overwhelmed pretty easily.
Three is a notoriously tough age. It's ok to let your child know that tantrums, but also use this time as an opportunity to teach her how to get what she wants and needs by being polite and nice. Maybe she could help you in some way with the oatmeal, or whatever she is asking for? Then she won't be distracted or forget that she asked for oatmeal.