Who answers the three questions i.e., what to produce, how to produce and for whom to produce in a "Mixed" economic system?

Both 1 and 2

In a mixed economic system, the government and the market together answer the three questions of what to produce, how to produce and how to distribute what is produced. In a mixed economy, the market will provide whatever goods and services it can produce well, and the government will provide essential goods and services which the market fails to do.