I have a solar panel that puts out 18V - 20V at about 1A. Is it possible to decrease the voltage to 2V and as much as 3A?

 

18v @ 1A = 18 Watts.
2v @ 3A = 6 Watts.
Yep, completely do-able - and you'll have power to spare, even if the converter isn't very efficient.

 

http://focus.ti.com/docs/toolsw/folders/print/switcherpro.html

There are hundreds of chips to choose from, and the TI switcherpro tool will give you the inductor, switches, feedback network, and layout for success.