Here's an example from equivariant homotopy theory. [Blumberg and Hill](https://arxiv.org/abs/1309.1750),
studying multiplicative structures on equivariant spectra, defined a functor from the (homotopy) category of
$N_\infty$-operads to the category of indexing systems, and conjectured that it's an equivalence of categories.

Three teams of researchers proved this conjecture at around the same time!
[Rubin](https://arxiv.org/abs/1705.03585) posted his proof in May 2017, followed soon after by [Bonventre and
Pereira](https://arxiv.org/abs/1707.02226) and [Gutiérrez and White](https://arxiv.org/abs/1707.02130), who posted
theirs on the same day in July. The latter two groups were aware of each other, and deliberately synchronized.

In this case, having three proofs is good: the proofs are different, and each proof has different advantages and
disadvantages. Moreover, the papers do more than just resolve the conjecture; each has different additional
material.