Can someone help with this question?

Rank the following in order of computation time from quickest to slowest, with m and n being 100 diget random numbers.

1. factor m

2. find gcd(m,n)

3. find two "probable primes" larger than m with a probabilty error of less than 10^-8

The answer lists them as 2,3,1

why is this? i would of thought 3 would be slowest and 1 quickest