3400 is already at the nearest 100. Rounding to the nearest hundred means that the last two digits (tens and units digits) are replaced by 00 if they are between 1 and 49 (for example 5419 would be rounded to 5400), and if the last two digits are between 50 and 99, then the hundreds digit is increased by 1 followed by 00 (so 2456 would be rounded to 2500).
3400.
3400
3400
3400
3000
3426 rounded to the nearest thousand is 3000 because 400 is less than 500
round 90 to the nearest ten
3400
372 rounded to the nearest hundred is 400
300
3400
100