We investigated second-order nonlinear optical properties of noncentrosymmetric crystals built of oblate, trigonal molecules: triphenylamine and adducts (complexes) of arsenic triiodide with sulfur, AsI3.S-8, and antimony triiodide with sulfur, SbI3.S-8. The efficiency of powder second-harmonic generation (SHG) in triphenylamine and in the adducts was comparable to that for urea. However, the SHG merit factor derived from coefficients of the NLO susceptibility tensor obtained from measurements on single crystals, is about two orders of magnitude larger in the adduct crystals than in urea. The vector and septor (octupolar) parts of quadratic susceptibility tensors are discussed on the basis of semiempirical quantum computations for triphenylamine and the experimental results in the adducts.