Reading your post this morning gave me flash backs to my own children at that age... my oldest walked in on my (now husband) boyfriend and I when he was 3. We're not sure how long he was there, but he too, had delayed development. Through the years I have watched him and he's never displayed any behavior that would even show he remembered the issue (He's 12 now).
Then, just last year, my (now 4-yo) son made a comment regarding something he'd *heard* between us, and I was floored to say the least! His statement brought fears to my mind that he would act out in response to the situation he'd witnessed, but he, also, has not displayed any behavior toward remembering it -- and he is advanced intelligence for his age.
I wouldn't worry about trauma in your daughter's mind, she most likely won't remember what happened. She will most likely only remember the door flying at her, rather than what she witnessed. But comforting her for the door issue was the best thing you could do.
If, later in life she brings up the situation -- but not the door -- then you can discuss it with her, but my best advice would be just to leave the original issue alone until she brings it up.
Best of luck