I am from Singapore, which uses the American region for 3DSes and eShop cards. A friend bought me a 3DS from America when he went there. I set my NNID to Brazil to be able to use the eShop. Will I be able to use eShop cards from Singapore (US region cards) on the Brazilian eShop?
The Nintendo eShop cards are region-locked. Cards from the PAL region only work in PAL consoles, and cards from Japan only work in Japanese consoles et cetera.
Fortunately, in your case, you will be able to redeem codes from cards that were purchased from Singapore, because Brazil and Singapore both actually belong to the North America region, as stated in Nintendo Wiki.