They are standalone and they allow you to get started in them, immediately. But best course is to complete the main game once, then hearts of stone, then blood and wine. This is the natural order and the right level (character's) adaptation to the setting and its challenges...

What order should I play The Witcher 3 DLCs in?

Basically the recommended dlc order is heart of stone first before main ending and after plotting agains Radovid there are some very minor dialogue then after finish main campaign do the blood and wine because Cd project said it was the ending of geralt.

Which Witcher 3 expansion pack first?

On 7 April 2015, CD Projekt announced two expansion packs for The Witcher 3: Wild Hunt—the first expansion being Hearts of Stone and the second being Blood and Wine.