I would talk to the store you got them from. It sounds like the two good buckets are half fermented and the brown bucket has completed fermentation and is oxidized. Never combine wines until they are clarified, stabilized, and evaluated. Even still, I'd recommend bench trials to determine blending percentages for the desired style.
If the store won't work with you, I'd give the brown bucket a chance to settle a bit. I've found that whites always look darker in the tank/carboy than they actually are. You could thief a sample and examine it in a glass to give a more accurate evaluation. If it is brown (oxidation), I'd add some SO2 stat and try PVPP fining and see where that gets you. A tannin made for white wine may help prevent further oxidation but it won't remove what is already there. Do not attempt MLF on the brown bucket bucket.