For Behaviourism, learning takes place as a result of a series of external reinforcements (i.e., rewards) or punishments to elicited behaviours.
For Cognitivism, learning takes place as a result of internal mental processing of information (e.g., elaboration, rehearsal, semantic encoding) leading to reorganized mental structures (i.e., memories - e.g., schemas, scripts); external behaviours indicate internal mental processes.
For Constructivism, learning takes place as a result of internal mental processes that learners are actively constructing about the external world as a result of interacting (including social interactions) with the external world.