I am rewriting this for two violins but am having a little trouble with the very first measure. They look like some weird sort of triplet. How do I enter the notes into a music editor? I've never seen that kind of syntax!
Dom has it right. They'd just be written as triplets with only the top or bottom notes present. You could enter them into the music editor the way you'd do any other triplet, with or without the triplet mark.
So, as far as stuff like left hand pizz, (which my computer keeps trying to say is "left hand pizza") it is currently not a possibility in Musescore to specify left hand pizzicato, but you can certainly tell it to perform pizzicato and write left hand in in the notes. (See: https://musescore.org/en/node/12183)
Triplets can be input with the information present here: https://musescore.org/en/node/11186
Which looks like this:
Although, as Tim said, it's possible that you'd want to use 12/8 instead of this method.