[Merged by Bors] - feat(CategoryTheory/ComposableArrows): API for the composition of three arrows

[joelriou]

This shall be used in the formalization of spectral sequences #33842.

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• depends on: [Merged by Bors] - feat(CategoryTheory/ComposableArrows): isIso_iff #33879

[github-actions]

PR summary 2cdef43fea

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference
Mathlib.CategoryTheory.ComposableArrows.Three (new file)	448

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Declarations diff

+ threeδ₁Toδ₀
+ threeδ₁Toδ₀'
+ threeδ₁Toδ₀_app_one
+ threeδ₁Toδ₀_app_two
+ threeδ₁Toδ₀_app_zero
+ threeδ₂Toδ₁
+ threeδ₂Toδ₁'
+ threeδ₂Toδ₁_app_one
+ threeδ₂Toδ₁_app_two
+ threeδ₂Toδ₁_app_zero
+ threeδ₃Toδ₂
+ threeδ₃Toδ₂'
+ threeδ₃Toδ₂_app_one
+ threeδ₃Toδ₂_app_two
+ threeδ₃Toδ₂_app_zero

You can run this locally as follows

## summary with just the declaration names:
./scripts/declarations_diff.sh <optional_commit>

## more verbose report:
./scripts/declarations_diff.sh long <optional_commit>

The doc-module for script/declarations_diff.sh contains some details about this script.

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No changes to technical debt.

You can run this locally as

./scripts/technical-debt-metrics.sh pr_summary

• The relative value is the weighted sum of the differences with weight given by the inverse of the current value of the statistic.
• The absolute value is the relative value divided by the total sum of the inverses of the current values (i.e. the weighted average of the differences).

[mathlib4-dependent-issues-bot]

This PR/issue depends on:

• [Merged by Bors] - feat(CategoryTheory/ComposableArrows): isIso_iff #33879
  By Dependent Issues (🤖). Happy coding!

[jcommelin]

Thanks 🎉

bors merge

I wonder whether eventually all of this should be brought together in a simp-proc. But for now, let's merge this.

[mathlib-bors]

Pull request successfully merged into master.

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