Yeah IDK... I suspect that exact tipping point is up to a jury and which side has the most persuasive legal team don't ya think?

 
I guess you would have to define what constitutes a melody, does it encompass a compete music idea, was the intent to recreate that melody in a similar way, etc, etc.  In other words, rather open to interpretation and argument.
 

The jury in that case decided that 3 notes constituted copyright infringement.... 
 
"He's so fine"="My sweet Lord".... bang!!!... millions of dollars changed hands over 3 notes.
 
Do I think that's right? Nope.... but that's what the jury decided.  So, copy up to 2 notes safely, go to 3 and you're  on risky territory. However, it's good to note that BOTH songs in that case were really big hits.... so unless your 3 note copy song hits #1, I really don't think it would be an issue because there's no money to go after.
 
It's all about the money..... not the notes.

I don't disagree in some cases.  For a hypothetical.  Take the NBC theme. (C-A-F).  If another news organization decided to use the same three notes for their theme, that would be a blatant violation, and I say sue away.  If the same three notes were used somewhere in a song with a 10-12 note melody that sold millions of copies.  There would be absolutely no cause for a lawsuit.  You can't sue over notes, only over obvious theft of intellectual property.  
 
If you see the Hans Zimmer hashtag I posted about the Planets.  The Gustav Holst foundation actually sued Mr. Zimmer for the film score to Gladiator because the opening battle scene music was a blatant ripoff of "Mars the Bringer of War."  Even though the notes and rhythms were very different.  http://www.filmmusicmag.com/?p=638
 
I don't really know the outcome of that case, or if it is still in process.  

rumleymusic
If the same three notes were used somewhere in a song with a 10-12 note melody that sold millions of copies.  There would be absolutely no cause for a lawsuit.  You can't sue over notes, only over obvious theft of intellectual property.

Exactly.
 
 

This happens to me pretty often, most recently just a day or two ago. I was trying to come up with a folk ballad, and was happy with what I had after about an hour of work, only to recognize it as something I have already heard.
When this happens to me, If I DO identify the melody I try to go back further and find an even earlier use of the melody. I regard this as licence to use what I came up with. There's nothing new under the sun.
 
Back when I played gigs I had a fake book I bought that had this weird index in the back to help the performer identify tunes by it the notes went up or down or stayed the same. So "The Star Spangled Banner" might look something like:
D-D-D-U-U-U-U-D-D-D-U-U -S-S-U etc.
Pretty laughable. Us guys use to play "Name that Tune" trying to use it in reverse. Never once did any of us get one right.
 
Here are a couple of my heartaches, one for the tune and a couple for titles.:
 
A lullaby I wrote in the 70's (but never copyrighted) opens exactly like "A Whole New World" from Disney's "Aladdin". Made me feel bad I didn't have it registered, but then I recalled "I Left My Heart in San Francisco" and felt better.
 
I have a song called "Let It Go" copyright 2009, probably before the "Frozen" song of the same name. BTW I'm getting quite a few looks at it on SoundClick the last few days. Also, I just learned that Def Leppard had a "Let It Go" back in the early 80's. Mine doesn't sound like either.
 
I wrote a song called "Come Away With Me", only to learn there was a big country(?) hit of the same name not all that long before I wrote mine....but I felt better when I recalled the barbershop that opens with "Come away with me Lucille - In my merry Oldsmobile" from before you or I were born.
 
I have more that I can't even remember anymore. I don't worry too much about it.
 

I agree with other in that if it is not obvious, just go with it and see how it turns out.  You don't want to stop your creativity.  Someday, if a publisher listens and says it is too much like another song, then you'll know.  Or, if an artist records it and someone sues you later, then you'll know.  
 
The best advise if you are unsure is to ask, research, post a draft hear on the boards and ask.  That way at least you got feedback.  
 
Rimshot