In recent years gamma-ray astronomy, neutrino astronomy and cosmic ray observations have obtained impressive astrophysical results. Modern astronomy and astrophysics are also experiencing an exciting era as the Multi-wavelength, Multi-messenger and Multi-observatory observations (3Mo) are becoming mature due to the powerful detectors and advanced analysis techniques. Air shower detection at high altitude, including imaging atmospheric Cherenkov telescopes such as HESS, MAGIC, VERITAS and CTA in the future, large field-of-view particle detectors such as Tibet AS-γ, ARGO-YBJ, IceTop, MILAGRO and HAWC, have demonstrated great potential of high-energy gamma and cosmic ray observations. Data from current and future experiments at high altitude, as LHAASO in China and possibly other projects in the southern emisphere, will be a key piece in the 3Mo frame. The aim of this workshop is to review recent major results obtained in the gamma-ray astronomy, neutrino astronomy, cosmic ray and astroparticle physics, including relevant theoretical studies on acceleration scenarios. Special attention will be paid to the complementary aspects among ground-based experiments at high altitude and their connection with other underground and space-based observations.