noun, noun which may take the genitive case particle 'no'
three kinds; three varieties
Used to refer to three distinct types or categories of something. Often appears in compound terms like 三種混合 (triple combination) or 三種の神器 (the Three Sacred Treasures).
This shop carries three kinds of cheese.
I received the triple combination vaccine.
三つ simply means 'three' when counting objects, while 三種 specifically means 'three kinds' or 'three varieties' of something.
Compound of 三 (three) and 種 (kind, variety). The reading さんしゅ is the standard on'yomi combination.