Why are the 13th, 14th, and 15th amendments referred to as the Reconstruction Amendment?

Because there were designed to ensure equality for recently emancipated slaves.

Answer=

The 13th, 14th, and 15th Amendments, known collectively as the Civil War Amendments, were designed to ensure equality for recently emancipated slaves. The 13th Amendment banned slavery and all involuntary servitude, except in the case of punishment for a crime.