1Ds3: Unusable above ISO 800ish. Superb ISO 50-200. Not a low-light camera and old tech.
5D2: Great range, poor AF system. Good IQ up through ISO 6400.
5D3: Superior of the 3, great AF system, auto ISO in manual mode, in cam Multi-exposre, in-cam HDR, etc.
I've owned all 3. The 1Ds3 was really superb at low ISO but that fell apart really fast. I never bothered to shoot above ISO 800. It's also old technology, with Digic 3 processors. You can spot-meter thru active AF and is a superior landscape and well-lit scenery/portrait camera. It is NOT better than the 5D3 though, even in these conditions.
The 5D2 was great IQ-wise, but not as good as the 1Ds3 ISO 100-400. Beyond 400, the 5D2 looked much better, and was usuable at high ISO values. Very poor AF system, and only has 9 points.
The 5D3 is the clear winner. It has an advanced AF system and you can do auto ISO in manual mode. The noise performance is a bit better than the 5D2. Also has the newest technology. You can use the 5D3 in ANY condition, whereas the above 2 would have to be more specific.
You need to decide what features you need. I wouldn't recommend the 1Ds3 however, because there is better tech available now.