to me “sea” doesn’t mean anything except for “general water region, smaller than an ocean”. I don’t think that there’s anything wrong with considering those three to be “gulfs”, but I think that people don’t usually think of them that way because unless you studied the geography carefully, you might not even notice that they actually are connected to the rest of the ocean.

by the way, fun fact: there is actually a sea that isn’t connected to the ocean and why we don’t call it a lake, I really don’t understand. but that’s the Caspian Sea.