As you can see on the title, what I recall correctly is, every Mangekyou Sharingan user must have awoken the 3 Tomoe Sharingan before awakening the Mangekyou. For example: Uchiha Sasuke, Uchiha Itachi, Uchiha Madara, and Uchiha Fugaku. All of these users awoke their 3 Tomoe Sharingan and then later on awoke Mangekyou Sharingan.
But just now I found a small loop hole: Obito and Kakashi. Before they awaken their Mangekyou Sharingan, their previous Sharingan only has 2 Tomoe. Here is the clip that I found.
As you can see below, both of their eyes only have 2 Tomoe before Kakashi kills Rin.
But after Kakashi kills Rin, they instantly skip the 3 Tomoe Sharingan and jump straight to awakening the Mangekyou.
How can they can skip the 3 Tomoe and instantly awaken the Mangekyou? Is this a small detail that the writer didn't explain?