I'm an engineer at Kinvey, and should be able to help ya out with this but I might need some more information.
Three things: Your query looks properly formatted, so I don't think that's the problem.
Secondly, Make sure your HTTP Method
is set to GET,
and finally, in the URL target
field you can let the website autocomplete /appdata/:appkey/:collectionName:/
for your specific app and collection, and all you want to do is append your query ?query={"_id":"3"}
to the end of that autogenerated URL.
Then, click Send Request.
If that doesn't work, please post a comment about what the result is and I can follow up!