Abstract: The development of white-light phosphors via Eu3+/Tb3+/Tm3+ co-doping remains challenging due to imbalanced energy transfer and site-selective incorporation. In this work, we investigate the luminescent properties of BaBi1.70 − xEuxTb0.15Tm0.15B2O7 (x = 0.05, 0.10, 0.15) solid solutions synthesized by solid-state reaction. Powder XRD confirms homogeneity of phosphors and a monotonic decrease of unit cell parameters, consistent with Bi3+ → Eu3+ substitution. Based on experimental data, the miscibility limit for the triple-doped system is estimated at x = 0.475, which is higher than that for single-doped BaBi2 − xEuxB2O7 (x = 0.45), indicating an expansion of the isomorphic miscibility range upon co-doping. Under 295 nm excitation, luminescence is dominated by intense red emission from Eu3+ (614 nm), while green (Tb3+) and blue (Tm3+) components are virtually absent. Excitation spectra at 614 nm show no features attributable to Tb3+ or Tm3+, and CIE coordinates remain in the red region (x ≈ 0.647, y ≈ 0.345). Although white emission was not achieved, this study provides some insights into the role of crystallographic site preference and compositional deformations in multi-ion-doped phosphor design which is essential for rational development of efficient UV-convertible white-light materials.