An interesting resource can be found at TI.com, Texas Instruments. On the home page is a link to their WebBench Designer. Click on the LED tab and input your requirements. You need to register to use it but it is a very useful tool for comparing designs. WebBench will give you all the controls for simulation, operating values, optimization, BOM, schematic and charts. Don't expect magic bullets though.
One solution given for 3A at 6.2V from a 9V-12.6V source involved an LM3401 buck controller IC. This is very similar to Maxim part mentioned several posts back.
Wight, I am not sure how to interpret the specification for a minimum 3V requirement for line regulation where the source voltage on Vdd can drop to 2.7V.
I think that the “Line Regulation” item has tricked you a little bit. The maximum amount regulation can change is 3mA/V under the test conditions specified: “VDD= 3V to 6V, VOUT=0.2V”. I interpret that as a delta of 9mA between putting Vdd at 3v and putting Vdd at 6v. Note that they do not include info for <3v, probably because the situation rapidly looks worse below 3v. Note “Supply Voltage” from the table above that one (top table on Page 3), spec’ed as 2.7v min.