Welcome to our site! EDAboard.com is an international Electronic Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.
You could use a linear regulator, such as the LM317, with an external pass transistor. See the app notes for the LM317 at http://www.national.com/ds/LM/LM117.pdf.
However, keep in mind the power (23.4 Watts) that the pass transistor must dissipate. You would need a fairly large heat sink to keep the chip temperature of the pass transistor less than 150 deg C. A more efficient solution would be a buck switching regulator. You can find lots of reference designs for this type regulator on the web.
Here is a boost regulator circuit using LM317. You may use this circuit to get the 4.2Volts, 3A. Adjust the variable resistor to get the required volatge at the output. Hope it helps you. Do not forget the "helped me" button. Good luck.