Upgrading to v3 and struggling with production previews, but successfully resolved with help from community.

document: {
    // prev is the result from previous plugins and can be composed
    productionUrl: async (prev, context) => {
      // context includes the client an other details
      const {client, dataset, document} = context

      if (document._type === 'post') {
        // you can now use async/await 🎉
        const slug = await client.fetch(
          `*[_type == 'routeInfo' && post._ref == $postId][0].slug.current`,
          {postId: document._id}
        )

        const params = new URLSearchParams()
        params.set('preview', 'true')
        params.set('dataset', dataset)

        return `<https://my-site.com/posts/${slug}?${params}>`
      }

      return prev
    },
  },
})

document: {
    // prev is the result from previous plugins and can be composed
    productionUrl: async (prev, {document}) => {

      if (document._type == "page"){
    return `<https://sitename.netlify.app/page-preview/${document._id}>`
    }
    if (document._type == "product"){
      return `<https://sitename.netlify.app/product-preview/${document._id}>`
      }
    if (document._type == "post"){
        return `<https://sitename.netlify.app/post-preview/${document._id}>`
        }
  }

      return prev
    },
  },
})

 return prev