No, they will not.
Characters from Swap Force can not be used with previous Skylanders games.
gaming.stackexchange.com/users/4957/TZHX
Some of the characters will work for all 3 games.
Each package has a key that has green check marks to show which game it will work on.
However, I have not found any of the swappable figures that work on other games.
gaming.stackexchange.com/users/66474/Jessica
Some will work in the Giants.
Since we only have Giants, my nephew actually told me to check the back of the figure packages and it will show which games they will work in.
From looking, I haven't seen many Swap Force figures that work in Giants
but I did find that Super Gulp Pop Fizz which is a Swap Force character does work in Giants.
gaming.stackexchange.com/users/67554/Christine
Basically the characters that were re-released in swap force work in giants such as twin blade chop chop, knockout terrafin, eruptor, super gulp pop fiz, big bang trigger happy, ninja stealth elf, anchorsaway gil grunt, mega ram spyro, and thorn horn camo.
Those are the ones i have that work
but im
sure there's more.
obviously none of the magnetic swappable figures will work since they didnt exhist when giants came out.
gaming.stackexchange.com/users/71395/Brian
Yeah but only the cores will work!
The swappers won't work same as the cores in Giants work in spyros adventure but the Giants dont!
(Cores are the little ones!)
gaming.stackexchange.com/users/95787/Viv
I bought the Skylanders Giant starter pack for my son for Christmas.    Now he asking for Swap Force characters.  I read that the Giants characters will work in Swap Force, but will Swap Force characters work with the Giants game?    I really don't want to spend the $ on another starter pack, so I am hoping just to add some of the Swap Force characters.
Will swap force characters work with the giants game
gaming.stackexchange.com
<skylanders>
gaming.stackexchange.com/questions/144635
gaming.stackexchange.com/users/63071/Robin
Some characters work for all 3 games. Not many Swap Force figures work in Giants.  Certain Swap Force characters can work in Giants game.
The community states that Swap Force characters do work in Giants. While a user is implying the opposite.