I'm building an ecommerce store as well as for the items, you will find different colors and dimensions. Same with it fine to possess a ProductType table with 3 composite secrets to define a distinctive row joining to some Product table?

Product Table: 
Product ID(pKey), Category ID, ModelNumber, ModelName

ProductType Table:
Product ID(pKey), Size(pKey), Color(pKey), Quantity

Getting an amalgamated primary key made up of 3 foreign secrets (or even more) is okay.

When the composed key distinctively identifies a row, then its a great primary key.

If ProductType getting used for research it is not good use 3 composite key ,i suggest produce a column for make unique the kinds,simply because type is going to be utilized in many tables and putting these 3 column in every table does not look great, also u put Quantity in ProductType ,it is'nt ok!