I was wondering whether or not this is a "proper" technique but when i do arpeggios say a 3 string arpeggio, ill use my thumb to pluck the lowest note, my index to pluck the middle note and my middle finger to pluck the highest note.

my other question is my band and i have decided to cover a song that we all learned how to play a year ago, Pull Harder on the Strings of your Martyr by Trivium. at the time when i first learned it i was using a pick much like their bassist did on the song but now i play using only my fingers and i cant get one part of the verse riff were you have to trem pick the open d string (drop d tuning so low d). would it be ok to just play the open note at half speed or not really?
Ibanez SR 505
Brice Defiant 6 lizardburst (soon to have)
GK 700rbII
GK BXL 4x10
GK BXL 1x15
Boss GT10B (soon to have)
to the second question...why not just use a pick? maybe you're rusty but if you practice it a little you could just use a pick for that song...variety is good

also..there is no "proper" way..whatever way you think is best would be the way you should play it

personally i just use my index and middle and go down the strings
much more simple to me
Just practice, that's what I'd do in your situation.

And to the thumb question, I do that sometimes too. Depends on the speed and if I want notes to ring out.
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That arpeggio technique is a valid one that many bassists use.
For your second question, I've always had the opinion to use whatever method best suits the song: pick, fingers, thumb, home appliance...