Minimum number of queries required to query for 2 level, 3 level relationships in salesforce

In 2 level you mean a parent and children, you can do this in a single query, although it contains a subquery that counts as another SOQL query for governor limits purposes.
In 3 levels you need to use two queries, one of which would contain a subquery.

Subquery also counts against Salesforce governor limit.

Cheers!!!