That's just our opinion though, because if it's true, Sonic 3 Movie will include Metal instead of Shadow.
I doubt it.
For starters the ending of Sonic's second movie started the hype for Project Shadow.
And from what you told me, one of the factors why Sonic Generations remake has Shadow's name in the billing is to drum up (unneeded) attention for Shadow's movie debute.
But Sonic 3 also having Metal Sonic would kind of be pointless given why need two "fakers" for Sonic? If Metal Sonic was in Sonic 3 then he will probably be regarded no more than a tool to draw attention from Shadow. But again Shadow is suppose to look like Sonic for mistaken identity to begin with. It would just be confusing.
Maybe if Metal Sonic was the mid-credit teaser then sure.
Sonic Speed Simulator - Dont Fall Off A Grind Rail!
https://www.youtube.com/watch?v=3iRZQgQo-q4
God these videos are pathetic.
Plus, it's easy to just slowly grind up and down some of these rails without falling off. Boring as heck but it's a way to grind points for events I guess.