You can still try and open it.
If it is only using standard addons, it should open. In the free edition, you won't be able to edit and save it, but it still will execute.
If your Construct 2 project is using third-part addons, you can have third-part Construct 2 add-ons in Construct 3 by asking their authors to port them using the SDK.
Otherwise, check the Exchange Addon or this dedicated forum to see if a port already exists.
Finally you can try this beta converter.
Once you have a Construct 3 version of the add-on, you can install it using the add-ons manager.