feat(data/sum/basic): Shortcuts for the ternary sum of types (#13678)
feat(data/sum/basic): Shortcuts for the ternary sum of types (#13678)

Define `sum3.in₀`, `sum3.in₁`, `sum3.in₂`, shortcut patterns for pattern-matching on a ternary sum of types.

Co-authored-by: Bhavik Mehta <89f31ab0a4650ba7936b563fb45961e72c36eef7@gmail.com>





leanprover-community/mathlib,leanprover-community/mathlib