Three Idiots

Poorva Mimansa is more atheistic than both Buddhism and Jainism.


Let us see Jainism do not entertains concept of god and Buddha remained his silence both for god and deities.

Later Buddhist rejected god but not deities, Jainism do believes in deities. But in Jainism these deities are not Siddhartha and can only bestow material gains. In Mahayana even god can be believed, and in Hinayana and even in Theravada deities are not rejected although here not much importance is given. 

That, in Buddhism deities are symbolic is a modern interpretations or propaganda , a more traditional Buddhist believes in the ontological existence of deities. although deities are not so important in Dhamma.
In Poorva Mimansa, the earliest document we have is Sabara Bhasya, a commentary on Mimansa Sutra attributed to Rishi Jaimini. Here only, the ontological existence of deities is refuted when Sabara writes , 'the vedic gods are mere words and sounds ' and 'there existence is not outside of vedas ' .No modern propaganda , no atheistic interpretations done as done to Adwaita Vedanta. But the earliest resources itself is clear on its stark atheism. Later Mimansak remains doubtful over deities and Mimansak like Kumarila and Prabhakara wrote extensively to refute the concept of god but maintained silence over deities.

In a nutshell Mimansa was more atheistic in origin over  Buddhism and Jainism  Today Vedanta is more famous among Vedics and many too find Adwaita if they leave Buddhism and Jainism , more interesting than Mimansa, not that mimansa is lesser than any other.