The poem, as written, consists of six stanzas, each with four lines. This structure is deliberate and contributes to the poem's overall impact. The poem's theme of creation, the existence of good and evil, and the power of the divine are explored through the contrasting images of the Tyger and the Lamb.
The inclusion of a third stanza would disrupt this balance and potentially dilute the poem's message.
It's important to remember that Blake was a highly intentional and meticulous poet, and it's unlikely he would have left out a stanza accidentally or without a specific reason.
Therefore, it's best to enjoy the poem as it is, appreciating its powerful imagery and thought-provoking themes within its six-stanza structure.