Okay, but what if [A-] is much larger than 2[B2-] which is much larger than [C-], would it be okay to exclude just 2[B2-] from the equation, even though [C-] is much smaller than 2[B2-]?
I think it
may work but you should try some cases to check. It is interesting as a mathematical question but not as a chemical one - if you can remove the larger, you can remove both.
The way I think it might work out can be visualized by considering, say, a number like 10,203. 10,203 = 10,000 + 200 + 3 and 10,000>>200 and 200>>3 in my example. Now let's say we want to approximate this in its parts - if we say 10,203 ≈ 10,000 then that is removing both of the smaller ones. If we remove 3, as is a logical step, this is a "very good" approximation because 10,203 ≈ 10,200 is a very good approximation. If we instead leave 3 and remove 200, we get 10,203 ≈ 10,003 - still a working approximation, better in fact than removing both, but nowhere near as good as just removing the smaller.
If you do try it with some numbers and proper simultaneous equations I would be interested to hear what you find.