the numbers we need to use only once are 7, 8 and 9 thus we need to find a 3-digit even number greater then 800.

the 3-digit number can not start with 7 since in that case it will be smaller then 800

this leaves us with following options:

1)8 is the first digit

the last digit must be even number but since 7 and 9 are odd number then 8 can not be the first digit

2)9 is the first digit

the last digit must be even number from {7, 8} thus the last digit is 8, and 7 is left to be the middle digit

the number is 978