Economic growth is not . . . . . . . . sufficient condition on its own to ensure . . . . . . . . increase in . . . . . . . . economic welfare.

The first and second blank shall take the indefinite articles 'a' and 'an' respectively as 'sufficient condition' is adjective + singular countable noun and starts with the consonant 's' and 'increase' is a noun complement here.
The third blank won't take any article as 'economic welfare' is uncountable.