I have to increase current but decrease voltage parallely. Can I do it?
You need to explain in more detail what you want to do.
Current through what? voltage across what? change how? what does "parallely" mean in this context?
What are you trying to DO?
I have 3v motors which get its input from the Arduino digital pin 12 of my Arduino Mega ADK. But the output is 5v. I have to decrease voltage. The Arduino provides around 20mA. Now I have to increase current. What exactly should I do?
Use a separate 3v supply (with ground tied to the Arduino) to drive the motors (it's good practice not to power motors off the same supply as digital electronics anyway, when this is convenient, and the 3.3v supply from the Arduino's 3.3v pin probably can't supply enough power.
Use an NPN transistor or MOSFET to switch the low side of the motors. Don't forget the diodes across the motor terminals (band toward positive side) to clamp the back EMF.
Alternatively you can drive the motor from 5V through a transistor, and use a PWM pin set to a value of 153 with an analogWrite call. This will keep the power dissipation in the motor down to what you would get with just 3V.
You can use a drop down regulator for the voltage but the increasing current will depends of what kind of power supply/bat. you use.
You need to understand that you can never power a motor from an Arduino I/O pin, or from its regulator output.
You use Arduino pins to control a transistor which switches power to a motor from some suitable power supply.