In 3rd Strike, Shin Akuma doesn't normally appear in the game, with the only final boss being Gill (and the game's secret fight featuring Q instead). However, leftover data from his 2nd Impact incarnation still exists in 3rd Strike and, with the help of cheats, you can either play as him or fight against him.

Despite being unused, he's actually suprisingly finished. In the time I spent fighting and testing him, I only noticed a couple of oddities:

His aerial Tatsu seems to miss some animation frames, which are replaced by glitched graphics in-game (during the parts where he's missing animation frames he also seems to be invincible).
When doing his aerial Tatsu to Shoryuken combo, he will perform the Shoryuken before he even touches the ground after the Tatsu.
His SAII (Messatsu Goshoryu) is extremely glitchy, as it freezes him in place for a couple of seconds when he initiates it and he also gains a lot less height with it.

He also misses his Kongou Kokuretsuzan Super Art, since it was introduced in 3rd Strike and Shin Akuma is based on his 2nd Impact incarnation. Finally, he also has his very own stage, but it's basically the same as regular Akuma's stage, but with a red moon and a slightly reddish tint (also, the bird in the background has blue eyes instead of red ones for some reason).

Hope you enjoy!

Source for border: imgur.com/gallery/gvtB4GV