Create text file and write one that already exist

So in my program their is a Random number of 3 digit ..

I wants that in each Random number lets say I have 289 I will create a new Text file (only at the begining ) and write the number

at the next Random number my program will see that I already have a text file Enter it .. go to the bottom and write the next number ..

at the end I want to see something lets say like this : 291 356 854 511 145

Thanx in advance