what scale should i use here?

i know the question is a little basic but I know just enough theory to help me write songs/jam to an extent

anyway, im trying to figure out a bassline for a song im writing but Im not sure which scale im even playing in. I tabbed out the chord progression below if that helps. Mainly im confused with the first three chords. I realize I could play open notes and what not but what Im looking for is a more....complicated/musical way of playing things.

Also if this makes no sense to people, sorry bout that



Drop D

~/- are just place holders for formatting

B~~~10
G~~~~-11 7 6----2
D 12 12 12 9 9 9 4 4 5
A
D 12 12 12 9 9 9 4 4 5