@AseahawkfanCan I use Overchannel to cast 3rd level spell [I]fireball[/I] using a 7th level slot? Seems to read that way.
It won't work, since that fireball is 7th level. The level of a spell matches the level of the slot used to cast it (PH, 201).

— Jeremy Crawford (@JeremyECrawford) May 14, 2015