Non-conventional food plants (NCFPs) are edible species that remain underutilized in the human diet despite being potential sources of phenolic compounds. This study characterized the phenolic composition and antioxidant activity of three botanically distinct NCFPs: sorrel (Rumex acetosa L.), nasturtium (Tropaeolum majus L.), and lamb’s ear (Stachys byzantina K. Koch.). We evaluated how extraction conditions influence the recovery of these compounds. We compared ten extraction protocols that differed in solvent, acidification, and ultrasound assistance. Total phenolic content and antioxidant activity (ABTS, DPPH, FRAP) were determined spectrophotometrically, and UPLC-DAD quantified eleven individual phenolic compounds. Recovery depended on both the protocol and the plant matrix. The highest total phenolic contents were obtained with ultrasound-assisted acidified methanol extraction, reaching 3540.73, 3940.83, and 4497.19 mg GAE/100 g DW for Rumex acetosa, Tropaeolum majus, and Stachys byzantina, respectively; however, the integrated phenolic analysis revealed species-specific responses. Chlorogenic acid was the predominant compound across all species and protocols, indicating that these plants are relevant dietary sources of hydroxycinnamic acids. Principal component analysis explained 89.5%, 82.8%, and 75.3% of the total variance for the three species. These findings support valorizing these edible NCFPs as functional food sources and highlight the need for matrix-specific analytical characterization.