A quantum calculation of the third virial coefBcient of a two-dimensional gas obeying fractional statistics is performed by diagonalizing the three-body Hamiltonian in a harmonic-oscillator potential with a large fermionic basis. The numerical results for the virial coefficient A3 as a function of the flux strength n (n = 0 for fermions, n = 1 for bosons) are reliable in the range 0 & n & 0.25. A3 increases quadratically with n for very small a, but higher powers contribute substantially for large PACS number (s): 05.30. -d, 03.65.Ge, 05.70.Ce, 74.65.+n In two-space dimensions, particles may obey fractional-statistics [1, 2], being neither bosons nor fermions.The equation of state of such a fractionalstatistics gas is of considerable current interest [3 -14]. The second virial coefficient of such a gas was calculated analytically by Arrovas et aL [3], and showed a cusplike behavior at the bosonic end. Although some interesting properties of the third virial coefficient have been derived analytically [5 -10], its actual evaluation is a difficult and challenging problem. Recently, de Veigy and Ouvry [14] have calculated the third virial coefficient perturbatively to order n2. In this paper we report the first fully quantum mechanical calculation of the virial coefficient by diagonalizing the three-body Hamiltonian in a large basis set. The calculation is successful in determining its behavior over a limited range of the statistical flux parameter.It is convenient, in calculating the properties of a system obeying fractional statistics, to regard it as a collection of either bosons or fermions, and use a statistical gauge interaction to generate the statistical phases [2]. In this paper, we use the fermionic basis, and the strength of the statistical interaction, denoted by n, may vary from n = 0 (fermion) to n = 1 (boson). The following important properties of the third virial coefficient As(ti, ) are already known.(a) The lowest power in As(a) is at least of order n (Refs. [6, 7]). (b) As(n) is symmetric about n = s (the semion), i.e. , As(n) = As(1 -o'. ) (Ref. [8]).(c) The eigenvalues of the three-body system that vary linearly with n do not contribute [9, 10] to As(n).Out of these, particularly useful for our purpose is the result (b) relating to the symmetry about n = s, which implies that it is sufficient to calculate As(n) numerically in the range 0 & a & 2. A direct quantum calculation of As(n) entails an evaluation of the three-body partition function Zs(n) in a box of dimension L or a harmonic oscillator with spacing h~, and evaluate As(n) in the limit A/L or huP~0, where A is the thermal wavelength and P is the inverse temperature (in units of the Boltzmann constant). Such a calculation involves a knowledge of the (nearly) exact eigenvalues of the three-body system to very high excitations (to be quantitatively specified later). Previously, only the very low-lying spectrum of the three-body harmonic-oscillator system had been calculated [15,16]. Following the perturbative method of Ref.[17], Sporre, Verb...