I'll do this in reverse order:
3. 'Workforce' is not an uncountable noun. It is a singular noun referring to a group of people. That's why it needs an article.
2. 'Productivity' is part of the noun phrase 'the productivity of the workforce'. This refers to a particular productivity i.e. the productivity of a certain group of people. That's why it needs a definite article.
1. Production is an uncountable noun, so it can occur without an article. Here it refers to production in general, which is why there is no article used.