Recently, advances in laser techniques made it possible to cool neutral atomic Bose gases below 1μΚ, and "Bose-Einstein condensations" have been observed, where a huge number of atoms occupy the lowest, ground-energy level. Although such a ultracold gas constitutes a many-body problem, its physical properties depend strongly on the interactions between two atoms or on those among three atoms. In the seminar, I will treat the problem of three-body collision, which is known to be one of the most difficult ones, and explain how it is possible to solve the problem numerically in quantum mechanics. I also treat the three-atom problem for Fermionic atoms and discuss the differences with the case of Bosonic atoms.