How to amplify current but not the voltage?
You take your “3V / few mA” signal, and feed it directly into the Base of a NPN transistor. The Collector of that transistor goes directly to +V of your power supply, the Emitter goes to the load and the other “end” of the load goes to 0Volt. You have to choose a transistor with a collector-emitter current Ice of >/= 3Amp. and your power supply must be able to handle that current, too. You may have to use a so-called Darlington transistor, depending on how much your “few mA” are…. Scroll down to the bottom of http://hyperphysics.phy-astr.gsu.edu/hba…
