Here is the fruit of Fendler's buckbrush (Ceanothus fendleri). Its width is about 4.8 mm. It's divided into 3 seed capsules roughly forming a rounded equilateral triangle. The fruits ripen in August and September, and when dry, the capsules explode, scattering the seeds.
Equipment used: Canon EOS Rebel T5i with a standard zoom lens (EF-S 18-55 mm) on a 7 mm long extension tube.