Choose regulator for 12V to 5V with 3Amp
How to select your DC-DC... To go from 12V to 5V you need a buck converter, so let's check that in the search engine. Now, for the other criteria...


Ready made or DIY?


That's the first question to ask, especially if you're not familiar with designing DC-DC converters. There are a lot of small board-mount modules available, for example this one. Search for "PoL (point of load) DC-DC" or "board mount" etc. You'll need a non-isolated buck. A ready-made module can be a very good idea for a small production run when you don't want to waste time on designing a DC-DC.


Synchronous or asynchronous rectification?


From 12V to 5V, duty cycle will be 5/12 = 0.42 thus a converter with asynchronous rectification will have a diode conducting 42% of the time, wasting 3A * about 0.5V * 0.42 = 0.625 Watts. A synchronous rectification converter will be a bit more efficient, it might waste about 0.3W instead in the bottom FET (wet finger in the wind calculation with 40 mOhms FET and switching losses equal to conduction losses). Since the difference between synchronous and asynchronous is less than 1.5% efficiency in your case, if asynchronous is cheaper, no problem, just use that. Synchronous becomes very useful at very low output voltages like 1.2V though.


Internal switch vs external?


An external FET means you can pick any FET you want, but it is an extra part and extra cost. However, an IC with an internal MOSFET will cost more than an IC without it. If uses synchronous rectification, then it is 2 FETs, or a single package dual FET. So you need to add the price of extra parts if you use a chip without internal FET(s).


Frequency


Higher frequency shrinks the inductor and the input/output caps, and reduces core losses. It also increases switching losses. 


Other features


Like efficiency at low load, losses at very low load or no-load, "power good" output, ENABLE input, input voltage range (do you want it to go down to 5V?), how much noise/ripple you can tolerate, EMI, etc.

Knowing this, just input your requirements into webench and check if it generates designs you like. You can set it to optimize for cost, footprint, or efficiency. Of course, it wants you to buy TI chips, but nothing stops you from using it as inspiration and using another similar chip from another supplier.

The first suggestion is this chip which looks pretty good. Note if your "3A max" load spends lots of time drawing 3A, it may be beneficial to select a chip with peak efficiency at 3A, and that would be a chip capable of a bit more current, perhaps a 4A chip. The 3A chip's efficiency peaks at 1-2A.