This theoretical study reports calculations on the fine and hyperfine structure parameters of the metastable X-3 Sigma(-)(sigma(2)pi(2)) state of ClH2+ and BrH2+. Data on the repulsive FH2+ system are also included for comparison purposes. The hyperfine structure (hfs) coupling constants for magnetic (A(iso), A(dip)) and quadrupole (eQq) interactions are evaluated using B3LYP, MP4SDQ, CCSD, and QCISD methods and several basis sets. The fine structure (fs) constants (zero-field splitting lambda and spin-rotation coupling gamma) and electron-spin magnetic moments (g-factor) are evaluated in 2nd-order perturbation theory using multireference CI (MRCI) wave functions. Our calculations find for Cl-35 of ClH2+ A(iso)/A(dip) = 110/-86 MHz; eQq(0) = -59 MHz; 2 lambda = 20.4 cm(-1); g(perpendicular to)(v = 0) = 2.02217; and gamma = -0.31 cm(-1) (to be compared with the available experimental A(iso)/A(dip) = 162/-30 MHz). For (BrH2+)-Br-79, the corresponding values are 300/-400 MHz; 368 MHz; 362.6 cm(-1); 2.07302; and -0.98 cm(-1) (experimental 2 lambda = 445(+/- 80) cm(-1)). We find g perpendicular to(ClH2+) to increase by about 0.0054 between v = 0 and 2, whereas the experimental effective g-L changes drastically with vibrational excitation. Nuclear quadrupole coupling constants for halogen atoms X are found to be as large as corresponding A(dip)(X)'s, indicating that both terms may have to be included in the Hamiltonian used to interpret XH2+ hyperfine spectra. A novel finding relates to the bound character of the 1(5)Sigma(-)(sigma pi(2)sigma*) state in FH2+, as already known for ClH2+ and BrH2+, but having a deeper potential well D-e approximate to 4000 cm(-1) (versus 1000 cm(-1) in the heavier radicals). Vertical ionization potentials for formation of XH3+ trications are also discussed.