In my opinion, DOF and ISO are a small problem; BORING LIGHTING is the main problem. Could be made so much better with more interesting lighting. Main/fill/fill is so bland - suitable for corporate headshots and family photos, but not for a band on a stage.
Were it me, I'd try to emulate stage lighting:
- On a tall stand, at least a foot above the tallest performer's head.
- Far away - at the back of the room, or about ten yards away, whichever is more. More distance to from flash to subject group, means the vocalist in front will receive approximately the same intensity of light as the drummer in the back. (inverse square law).
- Bare, or with a dish-style reflector - short of an actual stage spotlight, a bare hard light will get you most of the way to a stage-spotlight look.
- Pointed directly at the vocalist's face
- If a zoomable speedlight, zoomed in such a way that it covers the whole stage
Additional lights - you can play around with these for a few different looks.
- With gels, blue or green or red. If in doubt, use a nice saturated blue. Put gels on all lights except main!
- Bouncing might work, it'll eat up a lot of power but that's fine, you just want to add some interesting fill. After taking the gel's transmission value into account, power should be a couple stops lower power than the main light. You don't want to "fill" like you would a boring business headshot, you just want to add some interesting color.
- Or, on tall stands at the back corners of the stage, shining down at the performers' shoulders, to give some separation from the black backdrop. (probably where I'd start)
- Or, on the floor at the back of the stage, shining up at the backdrop, also to give some separation and color interest. Think party-atmosphere uplights
- Or, just about anywhere EXCEPT for that boring "fill light that's half a stop below main."
Do a Google Images search for "band on stage" and take inspiration from those. Colors on almost every light! Hard spotlights! I acknowledge that you only have three lights to work with, but you can do a lot with just three lights and a couple colored gels.