EDIT: Found it myself by going through some of Tarantino's OSTs, I kinda felt that it was old schooly and played by guitar all of the sudden:

It's Monte Carlo Nights, I know it from the Jackie Brown OST. My sisters know it because I used to play that OST pretty often:

https://youtu.be/6e6xhnKh7k0?t=16

I have the first three beats of a melody in my mind, but cannot for the life of me place where I know it from. My two sisters also know the melody (and were able to continue it after I only whistled the first two beats), but don't know where it's from either.

Here's me whistling it first, then my sister humming it: https://clyp.it/z1ddi1ef

It sounds similar to the store music from Binding of Isaac, in fact that's why I'm asking about the melody, but it's 100% not that, as I didn't know that song until a few days ago and neither do my sisters.

It might be a soundtrack theme in a TV show, a movie, some song, game even, I dunno. No idea how recent either. I'm certain the melody wasn't sung, however.

Edit: Just a small addition, I'm relatively sure that the melody is at least for a moment the main melody of whatever I'm thinking of, and also pretty much the same as I whistled it, not a variation, playing in the background right from the beginning. Not 100% sure, but pretty sure.

Hope somebody can remember where it's from ...