How is it possible for nepal to have all three major belts of natural vegetation?

Nepal is a Multi climate country.

Explanation:

Nepal contains 3 geographical climates In mountain region, there is cold climate, In Hilly region there is moisturized and cool climate, however in Terai region there is hot and dry climate, so different types of vegetations depends on these three climate. So, all the Evergreen forest, Deciduous forest, & Coniferous forest depends on these 3 geographical areas

for example in mountain region there is grasses, shrubs, alpine flowers, mosses, and lichens

and in hilly region, there is Orange Uttis, Salla, rhododendron, etc.

Furthermore, on Terai region there is Mango Cycas pectinata, Gnetum montanum, Duabanga grandiflora, Calamus spp., Cythea pinulosa, Pandanus nepalensis, and Podocarpus neriifolius.