Mathematic volume question

[studiojb]

I have two main vocal parts singing the same line, each at -6.
I also have one harmony track at -3
Everything is panned to center.

I want to raise the vocal mix, but keep the same proportions.

(I know I could send them to an aux input and raise that, but I'm looking for a mathematic solution)

Question: If I raise each main vocal to -3, do I raise the harmony to 0, or would I need to go to +3 (since there's only one harmony track)?

Thanks for feeding my insanity.