Halide perovskites are a well-known class of materials with many interesting applications. Great attention has been devoted to investigating halide perovskites containing triple methylammonium (MA⁺), formamidinium (FA⁺), and guanidinium (GA⁺) cations. Despite presenting very good applied perspectives so far, the lack of fundamental information for this system, such as its structural, thermal, and optoelectronic characteristics, prompts a step back before any technological leap forward. In the present work, we investigate the physical properties of mechanochemically solvent-free synthesized GA_xFA_xMA_1−2xPbI₃ halide perovskite powders with compositions of 0.00 ≤ x ≤ 0.15. We demonstrate that the synthesis of the powders can be performed by a simple manual mechanical grinding of the precursors for about 40 minutes, leading to solid solutions with an only minor content of unreacted precursors. X-ray diffraction, differential scanning calorimetry, and infrared spectroscopy techniques were used to investigate the structure, tetragonal-to-cubic phase transition, and vibrational characteristics of the organic cations with increasing GA⁺ and FA⁺ contents, respectively. The band gap and Urbach energies, obtained from ultraviolet-visible spectroscopy analyses, ranged from 1.58 to 1.65 eV and 23 to 36 meV, respectively, depending on the composition. These parameters demonstrate a non-random variation with x composition, which offers the possibility of a rational composition design for a given set of desired properties, demonstrating potential for optoelectronic applications. Finally, the system appears to have adequately tolerated heating for 12 hours at 120 °C in an ambient atmosphere, indicating high thermal stability and low ionic conductivity, which are desirable characteristics for solar cell applications.