Explanation:

The National Skill Development Mission (NSDM) primarily focuses on the following key sub-missions:

1. Institutional training - This involves enhancing the training infrastructure and capacity to promote skill development.

2. Overseas employment - This focuses on preparing candidates for jobs abroad and enhancing their employability in global markets.

3. Leveraging of public infrastructure - This pertains to utilizing existing public infrastructure and resources to support skill development initiatives.

Based on the information, all three options listed can be considered sub-missions of the NSDM. Therefore, the correct answer would be:

Option: 3 (1, 2 and 3).