How do I combine multiple OpenAPI 3 specification files together?

I wrote a quick tool to do this recently. I call it openapi-merge. There is a library and an associated CLI tool:

• https://www.npmjs.com/package/openapi-merge
• https://www.npmjs.com/package/openapi-merge-cli

In order to use the CLI tool you just write a configuration file and then run npx openapi-merge-cli. The configuration file is fairly simple and would look something like this:

{
  "inputs": [
    {
      "inputFile": "./gateway.swagger.json"
    },
    {
      "inputFile": "./jira.swagger.json",
      "pathModification": {
        "stripStart": "/rest",
        "prepend": "/jira"
      }
    },
    {
      "inputFile": "./confluence.swagger.json",
      "disputePrefix": "Confluence",
      "pathModification": {
        "prepend": "/confluence"
      }
    }
  ], 
  "output": "./output.swagger.json"
}

For more details, see the README on the NPM package.