Krishnappa was sworn in by Governor Vajubhai Vala, who administered the oath of office to him at the Raj Bhavan.
Manju and Kulkarni, who were ministers of state for Animal Husbandry and Sericulture and Mines and Geology respectively, have been given the ranks of Cabinet ministers.
They represent Arkalgud and Dharwad constituencies respectively.
The expansion of the Siddaramaiah government comes nearly three months after a major rejig, which had triggered discontent over dropping of some ministers.
The Chief Minister, in a major revamp, had sacked 14 ministers and inducted 13 members into his Council of Ministers on June 20, which had led to discontent with Ambareesh resigning as MLA.