We present the third discovery from the COol Companions ON Ultrawide orbiTS (COCONUTS) program, the COCONUTS-3 system, composed of a young M5 primary star UCAC4 374 − 046899 and a very red L6 dwarf WISEA J081322.19 − 152203.2. These two objects have a projected separation of 61 (cid:48)(cid:48) (1891 au) and are phys-ically associated given their common proper motions and estimated distances. The primary star, COCONUTS-3A, has a mass of 0 . 123 ± 0 . 006 M (cid:12) and we estimate its age as 100 Myr to 1 Gyr based on its stellar activity (via H α and X-ray emission), kinematics, and spectrophotometric properties. We derive its bulk metallicity as 0 . 21 ± 0 . 07 dex using empirical calibrations established by older and higher-gravity M dwarfs, and find this [Fe/H] could be slightly underestimated according to PHOENIX models given COCONUTS-3A’s younger age. The companion, COCONUTS-3B, has a near-infrared spectral type of L6 ± 1 INT - G , and we infer physical properties of T eff = 1362 + 48 − 73 K, log ( g )= 4 . 96 + 0 . 15 − 0 . 34 dex, R = 1 . 03 + 0 . 12 − 0 . 06 R Jup , and M = 39 + 11 − 18 M Jup , using its bolometric luminosity, its host star’s age, and hot-start evolution models. We construct cloudy atmospheric model spectra at the evolution-based physical parameters and compare them to COCONUTS-3B’s spectrophotometry. We find this companion possesses ample condensate clouds in its photosphere ( f sed = 1) with the data-model discrepancies likely due to the models using an older version of the opacity database. Compared to field-age L6 dwarfs, COCONUTS-3B has fainter absolute magnitudes and a 120 K cooler T eff . Also, the J − K color of this companion is among the reddest for ultracool benchmarks with ages older than a few 100 Myr. COCONUTS-3 likely formed in the same fashion as stellar binaries given the companion-to-host mass ratio of 0 . 3 and represents a valuable benchmark to quantify the systematics of substellar model atmospheres.