First off you need to use the desktop version of construct 3 and save as project files not a c3p file.
Images, plugins, instance variables, animation frames, etc can only be changed by a single person or you will get merge conflicts.
You can give every member of your team a different eventsheet to write events on and it will not cause conflicts
A lot of things in the engine use incremental IDs so if you add something and someone else add something else there is a conflict and the project doesnt open anymore
You can fix it with a text editor but it's pretty annoying