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I've never tried that before. You may use a LM7805 and put a transistor so that it can handle the current that your want. LM7805 in parallel give it a try if the voltage regulator gets hot then forget about that.
I also don't know if 3 7805's in parallel will work. What you also can try ( in addition to Necolei) is adding a small resistor (0.1ohm / 0.25W )between each input of the 7805 and V+. I think I have seen that once.
But do you really need a linear regulator? OK it looks like its cheap, but also hot even if Vin-Vout is about 3V (the minimum you need). You disipate then 3Watts in total, big heat sinks or fan.
Why not using a smps? You only need a LM2576-5, a coil, a diode and 2 capacitors.
Efficiency better then 80% and very easy to build.
Hi, well you don't need at all to connect 7805's in parallel as its a voltage regulator and it will be providing you a constant 5V and the amount of current depend on what you want to drain, and conencting it in parallel will only cause the transistors to heatup and eventually burnout...
as the power supply is concerned...so for the 5V just use a single 7805 and this will provide you the 5 V and as the 3A is concerned so for that you only need to attach a current regulator LM2576 ic in series with the 7805 output in order to regulate 3A constantly...thus the final output will be 5V and 3A ...
another simple design can be done is by just using TDB2912...this is a 5V 3A regulator...so a single ic does the task... http://www.tranzistoare.ro/datasheets/134/373860_DS.pdf