Here is a link on fears in 3 year olds:
http://www.google.com/search?hl=en&client=firefox-a&a...
Did he ever actually see something in there in the vent? Sometimes, children "do" actually have a "real" reason for their fears.
Did you ask him why he is afraid of vents?
Is he "new" to sleeping by himself, or in his own room?
I do know that young children will go through and out of periods of being fearful of things...they also have nightmares too. My girl is 5 years old... and she's had her "fear" phases at night going to bed or being by herself especially at night when it is dark. But your son seems to be afraid whenver he is alone even during the daytime?
But, they also grow out of it, or at least the phase will pass. It's a fickle thing. Just provide some comfort and "understanding".... they need to know that someone "hears" them and their feelings. Confidence and independence will renew itself I'm sure... once he passes the phase. It did for my girl as she's matured. Just don't scold him for it... although I"m sure you don't. They are having new feelings about things, "scary" things at night, the boogy man etc.
For my girl, we just let her sleep with us at those times. But she did go back to her room too, and she's evolved, and off and on. We also gave her her very own flashlight... and let her keep it in bed with her... it gave her a sense of "control" and being able to operate it at will if she wanted. She loved that idea and being able to do that.
Perhaps, did he see something scary on t.v. perhaps? Or maybe someone told him a "scary" story? Or did someone do somthing scary to him? Sometimes, growing up, my older sister would tell me "scary" things, thinking it was funny and she'd think it was funny the way I reacted. Yah, not fun for the younger sibling...but well. But that can happen too.
Or is he is in daycare/preschool or anything, did anything happen there? Or is there is change in his life recently?
It's hard to really pinpoint "where" a particular fear originates from... but to a child, it can make sense. Maybe explore things with him, talk about it, his feelings, why, how come, what is a vent, what is does, why it looks that way, etc. Sometimes when a child learns how somethings works, it is less forbidding or scary to them. Maybe during the daytime... see and touch it with him... tell a silly story about it... make it seem less threatening....
Poor little guy... i remember being scared of the dark too when I was little. It passes.
Take care and hope this helps,
~Susan
www.cafepress.com/littlegoogoo