I dunno, it could very well have been. After all, the USA's Particle Beam in Generals could very well be some predecessor to the Ion Cannon, and the GLA in Generals could have evolved into NOD along the way.
No, it couldn't. The Tiberium universe of TD and TS began with Red Alert 1 [meaning Einstein is to blame for the Tiberium Wars], as at the end of RA, you see Kane knock out the Soviet Leadership. Red Alert 2 was a bastard-child universe produced to make money, and while it was actually a pretty damned good entry into the series, it completely screwed up continuity and effectively branched the RA universe off from the Tiberium universe. So really, i'm not entirely sure if RA is still a prequel to TD.
Basically, RA [most likely] takes place around the late 50's/60's or so [correct me if i'm wrong, i've not played the game in years], with TD taking place in the mid 90's, and TS & Firestorm taking place early 21st Century. C&C3 will take place few years directly after Firestorm, quite obviously. Generals specifically
states that the story takes place in, what, 2020? Considering the first war between GDI and NOD was raging in the mid 90's, and Tiberium had already spread across a large portion of the planet by 2010, it's pretty obvious that Generals might just be little out of place.
Like I said, different gameplay, different universe, not a ****ing C&C game.