Atomic properties of n = 3 levels for W47+ - W55+ ions (Z = 74) are systematically calculated using two different and independent methods, namely, the second-order many-body perturbation theory and the multi-configuration Dirac-Hartree-Fock method combined with the relativistic configuration interaction approach. Wavelengths and transition rates for electric-and magnetic-dipole transitions involving the n = 3 levels of W47+ - W55+ are calculated. In addition, we discuss in detail the importance of the valence and core-valence electron correlations, the Breit interaction, the higher-order frequency-dependent retardation correction, and the leading quantum electrodynamical corrections for transition wavelengths. Spectroscopic accuracy is achieved for the present calculated wavelengths, and most of them agree with experimental values within 0.05%. Our calculated wavelengths, combined with collisional radiative model simulations, are used to identify the yet unidentified 25 observed lines in the extremely complex spectrum between 27 angstrom and 34 angstrom measured by Lennartsson et al. [Phys. Rev. A 87, 062505 (2013)]. We provide additional data for 472 strong electric-dipole transitions in the wavelength range of 17-50 angstrom, and 185 strong magnetic-dipole transitions between 36 angstrom and 4384 angstrom, with a line intensity greater than 1 photon/s. These can provide benchmark data for future experiments and theoretical calculations.