We recently derived a quantization condition for the energy of three relativistic particles in a cubic box [1,2]. Here we use this condition to study the energy level closest to the three-particle threshold when the total three-momentum vanishes. We expand this energy in powers of 1/L, where L is the linear extent of the finite volume. The expansion begins at O(1/L 3 ), and we determine the coefficients of the terms through O(1/L 6 ). As is also the case for the two-particle threshold energy, the 1/L 3 , 1/L 4 and 1/L 5 coefficients depend only on the two-particle scattering length a. These can be compared to previous results obtained using nonrelativistic quantum mechanics [3][4][5], and we find complete agreement. The 1/L 6 coefficients depend additionally on the two-particle effective range r (just as in the two-particle case) and on a suitably defined threshold three-particle scattering amplitude (a new feature for three particles). A second new feature in the three-particle case is that logarithmic dependence on L appears at O(1/L 6 ). Relativistic effects enter at this order, and the only comparison possible with the nonrelativistic result is for the coefficient of the logarithm, where we again find agreement. For a more thorough check of the 1/L 6 result, and thus of the quantization condition, we also compare to a perturbative calculation of the threshold energy in relativistic λφ 4 theory, which we have recently presented in Ref. [6]. Here, all terms can be compared, and we find full agreement.