It's worth thinking a bit,

See, in n-digit number each place has 3 options only,i.e, 2,5 and 7.

so, for a 3 digit number u'll have how much possible numbers: 3*3*3 = 3^3

for an n-digit number, we'll have 3^n numbers possible.

so, all what we need is : 3^n >= 900.

i hope now you can easily find n. :)