I have these three converters in my box, seemingly the CA-1235 looks best (gives 3A at 5V).
I want to power either a Pi Zero W or ESP8266 from a 12V power supply as part of my project.
Any advice / other option would be appreciated.
A Schematic or Drawing of your project.
A Brief Explanation of the project.
What does "BEST" Mean ?
see.. this is why we need a schematic,
So.. You're planning to use a 12V supply and connect it to an L.L.C. that is rated for a Max
of 5V ??
like i said "what does BEST ,mean ?"
Let's see the project
I agree
Don't use anything where you have to adjust the output voltage with a trim pot
So you're just thinking biggest current is "best"? 
So how much current do you actually need?
Note that switch-mode supplies (which these appear to be) work best at "full" load - so one that's over-specified on current might actually be worse!
Other things to consider include:
- what is the tolerance on the input side?
- efficiency - especially if your 12V is a battery
- how "clean" is the output?
- how well-regulated is the output?
I use the MT3068 units a lot.
A voltmeter is required to set them up.
Always "dial it in" before applying it to a project.
I started a few years back just buying SEPIC converters, it has saved me from some problems in the past when the 12V dips below about 7V.