Given the description, it appears to be an GCF problem.
So that means we have to find the greatest common factor common to all 3 volumes:
I'll start by listing factors of the largest number, 1000:
1000
1 * 1000
2 * 500 -> That matches 500, so it's a factor of 500, but it's too large to be a factor of 250
4 * 250 -> Matches 250, so it's a factor of 250.
Having found a factor for both 1000 and 250, we can now check if 500 also has that factor:
500
1 * 500
2 * 250
If I interpreted the problem correctly, 250mL should be the answer.