Can I replace a 3A plug on a light fixture (that comes with 3A cable) with a 5A plug?
From the mention of 3A and 5A plugs, I'm guessing that you're in the UK (or a country that uses the same style plugs).

The only difference between a 3A and a 5A plug is the fuse, and the fuses in a UK plug are interchangeable, so to convert the 5A plug to a 3A plug, just remove the old 5A fuse and install a new 3A fuse.

Your fixture doesn't have an earth lead, meaning that it's double insulated.  Connect the two wires on your light fixture to the terminals marked 2 and 4 in this diagram:



(Image courtesy of the Wikipedia article on UK plugs)