Because the buck converter is not designed to provide 3 amps?
I am sorry for the vague answer, but you must realize that your question is quite vague and is withholding crucial details.
You say you are feeding it to a buck converter...is it something you built or purchased? If so can you provide a diagram, a model number, a datasheet, a link to a website?
You also don't mention what happens, for instance, if you load your setup with a lighter load, i.e a 20 ohm resistor.... can you make the output current vary by varying the voltage setting potentiometer?