Shin is a Uchiha

[Legendary Saiyan]

He's an Onion. Due to that, he just cries 24/7 and the MS bloomed.

 

[Jokule67]

Filler Filler and more Filler

 

[Insomniac Fueled Suicide]

3T Sharingan is stronger than his Mangekyo lol

I don't believe so granted in the long run I would agree, but for short instances no Mangekyo is better. Because Mangekyo has the powers of 3 tomoe plus you're able to use special abilities such as amatseru and stronger genjutsu on top of seeing extremely fast movements. So if it was a quick 1v1 battle Mangekyo is stronger but if it's a drawn out fight like 1v3 then yeah you would want to use less chakra and use the 3 tomoe then.

 

[Yatori]

Shin was created in a lab, right? Oro probably added uchiha dna to him and bam, MS.

This ^^

 

[Obito the supreme jin]

From Obito's laboratory which Orochimaru probably got his hands on since he experimented with him.