Final answer:
The hippocampus and medial temporal lobe structures associated with episodic memory were likely affected, leading to the patient's difficulty with forming new memories and recalling recent events.
Step-by-step explanation:
The regions of the brain that are likely to have been affected by the antibodies in the case described are those associated with episodic memory, which includes the hippocampus and related structures. Episodic memory involves the recall of specific events, situations, and experiences. The inability to remember the three-word recall test suggests that the patient's hippocampus or surrounding medial temporal lobe structures were compromised, leading to difficulties in forming new memories or recalling recent events, despite other cognitive functions remaining intact.