How to initialize an std string with a length

Question

How can I correctly initialise a string if its length is specified at build time?

#include <string>
int length = 3;
string word[length]; //invalid syntax, but doing `string word = "   "` will work
word[0] = 'a'; 
word[1] = 'b';
word[2] = 'c';

...in order for me to accomplish anything like this?

I'm doing this because I have a loop that copies characters from specific parts of another string into a new string.