decrease 32V 3A DC voltage to 28V 3A
From your question I think you want to step down your DC voltage. Now there are various ways to do so:
1) You could use a linear regulator which supports high current, Wikipedia page on linear regulators
2) You could use a switching regulator which is technically perfect for your job as it handles high currents effectively Switching Regulator.
If I were you I would have gone with a switching regulator. Why? Reasons are clear : 


You won't easily find linear regulator for that sort of voltages
Switching Mode power supplies are more reliable and very efficient.


I would suggest reading about this regulator and look for similar regulators for your purpose : Buck Boost converter

Good Luck!