how to select the right TVS / fuse combination?
No, You'll need slightly more input power than output power. It depends on the efficiency of the converter as a whole. For example if it has an efficiency of say 90%, then you'll need about 1.4A at 12V (16.8W) at the input to get 5V 3A (15W) at the output. The extra power is used by the components to carry out the conversion, there would also be loses due to heat. That's why you would want to select an inductor with as low a DCR (direct current resistance) as possible and MOSFETS with as low an Rdson and gate charge (getting both low usually leads to more expensive devices) as possible (in your case with this controller, the MOSFETS are embedded so no choice to be made).
For me, I would just follow the typical application circuit that some manufacturers are nice enough to give. Your IC has one on page 15 of the datasheet that shows a circuit for your requirements of 5v 3A output. You could just go with that.


(On a side note these guys are pretty great for showing so many different circuits for different output options, I don't usually see that many!).