Hi there!
Thanks for the screenshot. The problem is your Duration value. You set it to 3. But that value is not equivalent in seconds, is in frames. So, the opacity is changing very fast, almost instantly.
- 60 frames = 1 Second
- 3 Frames = 0,05 Seconds
If you want for the opacity to change in 3 seconds, you should use 180 as value for the Duration.
Let me know if it worked!