2 I ...(learn) only three poems for the exam so far.

Explanation:

The phrase "so far" indicates that the action of learning poems has started in the past and continues up to the present moment. The number of poems learned is already specified (three). The present perfect tense is used to connect a past action with the present result or to describe an action that happened at an unspecified time in the past but is relevant now.

Answer:

I have learned only three poems for the exam so far.