Na3R(BO3)2 (R = La–Gd) borates, grown with Na2O–B2O3–NaF flux, crystallize in the space group P21/n. Contrary to previously published data, the peaks in the 980(La)–950(Gd) °C range are attributed to phase transition process, while the melting process occurs at about 100° higher. Another feature of thermal behavior consists of the gradual decomposition of the compound into Na3R2(BO3)3. The end member in the decomposition series is Na2R2O(BO3)2, which originates under oxygen/water atmosphere. In addition, the components of the thermal expansion tensor and the dependings of the unit-cell parameters of Na3Nd(BO3)2 on temperature were defined. A series of Pr3+, Nd3+, and Sm3+-doped Na3Gd(BO3)2 (NGB) borates were prepared from mixtures of grown crystals by solid-state synthesis, and their photoluminescence properties were studied in detail. The excitation and emission spectra of NGB: Pr3+ and NGB: Nd3+ are comparable with similar compounds, and the relative intensity reaches the maximum at 1 mol% of Pr3+, Sm3+, and 10 mol.% for Nd3+ doping.