Not sure this will help with your ‘3 legged OAuth flow’ workflow requirements, but you might wanna check out Mibex Software’s nice walk through on How to use OAuth with Atlassian products - it’s using OAuth1 though and based on Bamboo, but applies to Bitbucket Server as well:
While Atlassian’s cloud solutions like Bitbucket Cloud make it very easy to use OAuth, the behind-the-firewall editions of Atlassian products like Bamboo Server make this significantly harder — yet not impossible. In this blog post, we will show you how you can access Bamboo’s REST API with OAuth1. To achieve this, we’ll make use of application links. Although we have chosen Bamboo here, the basic procedure is the same for JIRA, Confluence and Bitbucket Server.
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