No big deal, you'll get there, I just got through doing the exact same thing with my last neck build.
Are you sure it is not the bridge or the tuners or the shirt button against the back of the body or.
My first thought when all strings buzzed on frets 1-3 was if your action was too low at the nut end....
yeah, what Taff said, flat top frets will buzz, you have to recrown them, not saying this is your only issue, but you have to eliminate it, you may have it solved and not know.
And for the record, this is what most call "The fiddly Part" of git building, so don't let it dissuade you, my last git, I used what I thought was dang close to perfectly flat maple fret board, same for the oak neck, was careful installing my frets, and buzzzzzzzzzzzzzz, had to file them, and had some EXTREMELY flat frets when I was done, mostly all gone now after I re-crowned them, I've only have some buzz below the 12th
I noticed the saddle adjustment screws look like they are touching the strings. Does that matter as only the first frets are buzzing
Maybe, could you add a picture of what you described? not enough break angel over the saddle can also cause open strings to buzz and fretting part way up the neck could add just enough...