3D First Person Platformer Questions

How do you add 3D enemies, 2D Enemies, 2D Sprites, and Bosses (2D and 3D)

It would be very difficult to make a 3D platformer on Construct. There are some other free game engines you can use.

There is other free engine similar to Construct? Event-Action coding??

There is other free engine similar to Construct? Event-Action coding??

I don't know much about other engines, but I think there are a few that are to some extent similar to Construct.

How do you add 3D enemies, 2D Enemies, 2D Sprites, and Bosses (2D and 3D)

Sorry for the almost year late response. 3D enemies are nearly impossible to add without lots of issues, but you can use a 3D cube with all faces invisible except one which can be set to use a sprite. I believe this is also the only way to have a 3D object with animations. The same applies to bosses.