3399 rounded to the nearest 1000 = 3000
The next digit at the right of 3 is 3 which is less than 5, so retain 3.
If the next digit at the right of 3 is equal or more than 5, add 1 to 3 which will make it 4. (4000)
3000
3399.000
3000
1000
1000
1000
1000
176 rounded to the nearest 1000 is 0
1000
1000
0
1000