M. sylvestris L. is a synonym of M. longifolia:

herbaceous perennial plant with a creeping rhizome, erect to creeping stems 40–120 cm tall. Leaves oblong-elliptical to lanceolate, 5–10 cm long and 1.5–3 cm broad, flowers 3–5 mm long, lilac, purplish, or white, in verticillasters on tall, branched, tapering spikes.

Mentha spicata:

herbaceous perennial plant growing 30–100 cm tall, leaves are 5–9 cm long and 1.5–3 cm broad, with a serrated margin, flowers in slender spikes, each flower pink or white, 2.5–3 mm.

Mentha arvensis:

herbaceous perennial plant growing to 10–60 cm, rarely up to 100 cm tall, leaves in opposite pairs, 2–6.5 cm long and 1–2 cm broad, hairy, and with a coarsely serrated margin. The flowers are pale purple, white or pink in whorls on the stem at the bases of the leaves.

Thank you very much Dr. Alessandro Venditti for your kind information.

You're welcome.

Hi. Apart from the useful information given by Alessandro, you have a good key, full descriptions and nice drawings of some Mentha species in the volume XII of FLORA IBERICA (2010), published by the Real Jardín Botánico de Madrid (CSIC).