I do know some but I stopped reading your post in paragraph two when you say "at the end of the movie" cause I hate it when people tell me about the end of something. Sorry...
Sorry Stefan! Next time I will write ***POSSIBLE SPOILERS***. I only wrote that because it was the only part of the movie I could remember.
David J wrote:
The first might be "Doppleganger" where a man flies to a duplicate Earth on the other side of the sun.
The second might be "Zapped" Scott Baio becomes as a high school student who becomes telekenetic. I'm less sure of this one.
The third one doesn't ring a bell.
Doppleganger... that could be it, but I'm still not sure.
The 2nd movie I was thinking about did not have Scott Baio in it.
After thinking about it I think the radioactive element was "H128" and now I'm almost certain it must be "City Beneath the Sea" from 1971 which has an entry on imdb.com . Also it was a TV pilot, not a theatrical release. I must have seen this when I was very young.