The recommended power supply for the 3B is 2.5A
the first buck converter mentioned before is not powerfull enough to power a 3B (it might work when the Pi 3B is not doing anything but under high load you might get the dreaded “insufficient power” error.
The second one provides 3A and should be sufficient… unless when you want to play audio from your raspberry pi…
In that case i’d go for a 6A 12V power supply and a 5V isolated step down converter.
They are not cheap… but they provide 3A instead of 2A and if you want to play audio from your raspberry pi, it is better to go for an isolated step down converter… and I had to learn this the hard way… as with a normal 5V buck converter I had a ton of strange noises coming out of the speakers (and it wasn’t a ground loop). Several cheap buck converters and hours later it became clear that I needed an isolated one:
Look for a Mornsun VRB1205LD (there are cheaper ones out there, I just copied the first one):