I used this to make a really nice patio table. I think as long as you finish it correctly it would do just fine on a park bench, but that of course depends on the design we are talking about.
They're actually .75 x 2.5. I used a different Home Depot product that is 3.25 with rounded edges, but if you have a decent table saw, you could trim it to 3" and make it square. Unfortunately, I don't remember what it was and it's installed already so I can't see the label.
Yes. You can also cut it at whatever size you need it.