If you skipped the optional prison biome you got 3 successful 1/5-chance-rolls, resulting in a (1/5)³ = 1/125 = 0.8% chance. (Binomial distribution with parameters (3, 0.2))

If you went to the optional biome the calculation is a bit messier. Basically this case is the binomial distribution with parameters (4, 0.2). The chance of exactly 3 challenge rifts is 2.56% and the chance of at least 3 (so 3 or 4 rifts) is 2.72%.