Bicycle Research makes a tool for pushing dents like that back into place:



If a local bike co-op doesn't have it, you can improvise with a c-clamp and a block of wood to protect the non-dented side. Be aware that even using the tool you will be hard-pressed to achieve a perfect result. Perhaps this would be a motivation to rebuild the wheel with an aluminum rim? You will get much better wet braking performance and a small reduction in weight with an aluminum rim. The Sun CR-18 rim in 590mm ERD is a drop-in replacement for the OEM rims on those Raleigh 3-speeds, meaning you can even use your old spokes if the nipples aren't seized. The CR-18 is available in 32, 26 and 40 hole spoke drillings.