to purchase three reales' (eighths of pesos) worth of something (see Molina)
Alonso de Molina:
yeitica niccoa. comprar tres reales de algo. Alonso de Molina, Vocabulario en lengua castellana y mexicana y mexicana y castellana, 1571, part 2, Nahuatl to Spanish, f. 35r. col. 2. Thanks to Joe Campbell for providing the transcription.