Jamieson, Fausset and Brown on Isaiah 8:3

3. prophetess--perhaps the same as the "virgin" (Is 7:14), in the interim married as Isaiah's second wife: this is in the primary and temporary sense. Immanuel is even in this sense distinct from Maher-shalal-hash-baz. Thus nineteen months at least intervene from the prophecy (Is 7:14), nine before the birth of Immanuel, and ten from that time to the birth of Maher-shalal-hash-baz: adding eleven or twelve months before the latter could cry, "Father" (Is 8:4), we have about three years in all, agreeing with Is 7:15, 16.