Abstract: Medicinal plants are a valuable source of biologically active substances used to prevent and treat diverse diseases. The major classes of these compounds—alkaloids, glycosides, flavonoids, essential oils, tannins, polyphenols, and polysaccharides—exhibit broad pharmacological activities, including antimicrobial, anti-inflammatory, antiseptic, and antioxidant effects. Phytochemical screening of plant extracts plays a crucial role in identifying these compounds and supporting the development of new therapeutic agents. In this study, a spectrophotometric phytochemical screening was performed to evaluate alkaloids, tannins, flavonoids, phenolic compounds, steroid cardiac glycosides, and polysaccharides and the antioxidant activity of extracts from three medicinal plants from different families, milk-ripened oats (Avena sativa L.), madder root (Rubia tinctorum L.), and Scotch heather (Calluna vulgaris L.), was assessed. High-performance liquid chromatography was used to achieve a more detailed chemical profile. Both conventional solvents (methanol, ethanol, water-alcohol mixtures, water, acetonitrile) and a new class of solvents, deep eutectic solvents, were used as extractants. Ultrasound-assisted extraction proceeded at 40–80°C for 10–60 min. The optimal extraction conditions were found for the major compound groups: 80°C for 60 min with conventional solvents, 50°C for 30 min for Calluna vulgaris L. with deep eutectic solvents, and 60°C for 30 min for madder root and oats with deep eutectic solvents. The results demonstrate that deep eutectic solvents effectively extract the target bioactive substances under milder conditions, underscoring their potential as environmentally sustainable extractants in phytochemical research.