Sundering Stroke deals 7 damage divided as you choose among one, two, or three targets. If at least seven red mana was spent to cast this spell, instead Sundering Stroke deals 7 damage to each of those permanents and/or players.
You must divide the damage as you announce Sundering Stroke, even if you intend to pay seven red mana to cast it. If you copy Sundering Stroke, no mana was spent to cast the copy at all, so the copy will deal the damage as divided.
Each chosen target must receive at least 1 damage.
If some of the targets become illegal for Sundering Stroke, the original division of damage still applies, but the damage that would have been dealt to illegal targets isn’t dealt at all.
If an effect allows you to cast Sundering Stroke without paying its mana cost, you can’t choose to cast it and pay its cost unless another rule or effect allows you to cast it for a cost.
Sundering Stroke checks what mana was actually spent to cast it. If an effect allows you to spend mana “as though it were mana” of any color or type, that will allow you to spend mana you couldn’t otherwise spend, but it won’t change what mana you spent to cast the spell.