You would round 2,722 to 3,000 because the standard rule for rounding numbers is: if the number is 4 or less, round down; and if the number is 5 or more, round up.
The principle is the same for any large number, the only difference is that there are added digits or zeros.
So, because your question is to the nearest 1000th, you'd round up to 3000.
Kind of like this:
2,400 or less, rounds to 2,000
2,500 or more, rounds to 3,000.
Get it? :)