Medicinal plants have been essential in traditional medicine and continue to play a significant role in modern pharmaceutical and nutraceutical industries, particularly for their antioxidant properties. In this study, we examined the phytochemical composition and antioxidant activity of extracts from the leaves of Trichosanthes lobata Roxb and Uvaria narum Wall ex Wight, and the bark of Zanthoxylum armatum DC. Both alcoholic and aqueous extracts were analyzed for their polyphenol and flavonoid content using High-Performance Thin Layer Chromatography (HPTLC) and spectrophotometric methods. The alcoholic extract of Z. armatum bark contained the highest polyphenol content (216 ± 5.1 mg/g gallic acid equivalent), followed closely by the aqueous extract of T. lobata leaves (213 ± 1.9 mg/g). Regarding flavonoid content, the aqueous leaf extract of T. lobata was the most abundant (64 ± 5.5 mg/g quercetin), while the alcoholic leaf extract of U. narum contained 42 ± 3.5 mg/g quercetin. Antioxidant activity was evaluated through DPPH and nitric oxide (NO) scavenging assays. The aqueous extract of T. lobata exhibited the highest DPPH scavenging activity (89.3 ± 3.4%) at 200 µg/mL, comparable to ascorbic acid. The alcoholic leaf extract of U. narum displayed the highest NO scavenging activity (85.7 ± 3.2%). Additionally, the total antioxidant capacity (TAC) of T. lobata was 3.5 ± 0.2 mmol of ascorbic acid equivalents per gram, and its ferric-reducing antioxidant power (FRAP) was 456 ± 3.7 µmol Fe²⁺/g. These findings demonstrate the strong antioxidant potential of T. lobata, U. narum, and Z. armatum, highlighting the value of T. lobata leaves due to the presence of quercetin and gallic acid flavonoid as a promising source for the development of functional foods, nutraceuticals, and phytopharmaceuticals.