You can use the default tileset for the Tilemap object in any project (commercial or personal) as long as it is made with Construct 3.
No mention is required.
You are not allowed to redistribute or sell the tileset image as an asset (image) on its own.
You cannot claim copyright for it or a modified version of it.
You pretty much can do whatever you want with it as long as it is in a work made with Construct 3.