I rack from my primary onto sorbate/campden in a bottling bucket. I then backsweeten and add my clearing agent. So my wine sits in the bottling bucket for 3 days.
If i do a 3 gal batch in a 5 gal bucket, is that 3 days long enough to ruin my wine?
"Ruin" is a strong word. I wouldn't say it won't "ruin" it, but it WILL "impact" the flavor of it. That's certainly enough time and headspace to cause the beginning of oxidation, even with the campden tablets. It will get worse with time, so if the wine is consumed young it shouldn't really be all that noticeable.
As an example, think of this. You open a bottle of wine with dinner, but only drink half. Pop the cork back in it, and drink it the next day. The wine will be markedly different, and definitely not the same wine you opened. Oxygen does that, and does it quickly.
When the wine has sulfites added, it does prevent oxygen from binding with the wine, at least for a very short period of time (say, 24 hours or so). But there is still plenty of oxygen available in a bucket with such a wide headspace.
If this was my wine, I'd rack to a carboy to stabilize. Wait three days or more, and then sweeten. And then wait three more days (at least) for the wine to clear and not start fermenting again, and then I'd rack to the bottling bucket and proceed with bottling.
