hehe welcome to YC
What I have to say about Multiple parts writing - well, I find it a breeze because I am a pianist...so i see what you are saying there. However, I find it hard to write medium outside of the piano haha. If i intend on writing a trio for example, I will predominantely use the piano more than the violins and cellos. So what I do in that case then, is write down or record myself thrashing the piano, and then take it to my notation program (finale in my case) and see what happens. Most people would disagree with this creation process...but I find it works wonders. I can write for other instruments much better on finale because I can hear them.
What you should probably do then is (and I know this is gonna probably annoy a lot of people) - start small: write a piano part that really just uses the left hand to start with, like with a Alberti bass...and then play/write/sing a melody over the top...just randomly...and then write it down. This is a very unstructural way to write pieces, but for now...hear me out. So there we go, we have 2 parts...though they dont seem to be much, they will soon serve their purpose.
I wouldnt give the guitar 'chords' to strum throughout because then you will find it becomes to 'pop-py'. So perhaps play that melody on the guitar (the one you sung/playerd/wrote) and see how that goes. Then the violin can do things like 'echo a melodic motif' in the background.
BUT: FIRSTLY...get a figure happening in the left hand of the piano. Then a melody on one instrument (for example, guitar)...after you have written a pretty decent 8 bars of music - THEN sit down and think. Play it through a few times (this is where a notation program helps), and listen with your inner ear to where the violin could enter, whether it echo, or just pluck chords.
Basic structure, and its hard to instruct without blabbing (which i have done a fine job of), but hopefully that helps. Its really a thinking process to start with.
Then there is counterpoint...but start small, and work your way up. Write for the instruments you know best first...Piano and Violin, Guitar and Violin, Piano and Guitar, Two Violins...but always start small.
Again, welcome.
Nick Goodridge