I'm getting my grandson started on Skylanders for Christmas & know nothing about the story/game. I bought Giants & want to give him the Spyro figurine (which I'm having a hard time finding without also buying the Spyro's Adventure starter pack), but so far I have only found it from the Nintendo 3DS starter pack. Will he work on a PS3 platform or are the figurines platform specific?
Yes, the figures are cross-platform compatible.
Make sure you buy your grandson a Starter Pack for his specific platform (either Skylanders Spyro's Adventure or Giants), and not a Booster Pack, both kinds include them game and figure(s) but only the starter pack includes the required portal of power.
The Spyro figurine isn't available for purchase by itself, the regular one was bundled with the console starter packs and Dark variant was bundled with the 3DS starter pack, but the Legendary variant is available in a Toys "R" Us exclusive bundle. Some figures that were only available in level packs or bundles for the first game are now available separately, so Spyro might be available on its own soon.