Assuming a1, a2 and a3 are not all same and co-prime with 999, the answer will be 999.
N = 0.a1a2a3a1a2a3.....
1000N = a1a2a3.a1a2a3a1a2a3....
so 999N = a1a2a3
=> N = (a1a2a3)/999
Now either a1a2a3 and 999 are not co-prime, in that case there will be a smaller number than 999 (which can't be determined with the given data)
Or, a1a2a3 and 999 are co-prime, in which case 999 is the answer.