#JavaScript (ES7), 80 bytes

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    (a,b,c)=>((b*=a*c/((k=(b+c-a)*(c+a-b)*(a+b-c))*(a+=b+c))**.5)*(b-(k/a)**.5))**.5

[Try it online!][TIO-k8zvw2gv]

[JavaScript (Node.js)]: https://nodejs.org
[TIO-k8zvw2gv]: https://tio.run/##fY7BDoIwEETvfsm2bEFaCHqoR/9juxSDEGqEGP@@Nnji0kyyM5N5h33Sh1Z@j69NLaH3cbARCB2ysDcAJy1JrgAmC65gRUICF6RcciqcYrEHm7aUZNmm6hRMFf3bfiOHZQ2zL@fwgAE0GmxEuYX7@PU9GCFORyDN2OaBFrsc0OEV63OOuGBSDqi1QW0aNM3xlfgD "JavaScript (Node.js) – Try It Online"