The Wicher 1 and 2 were not very well connected. They had totally different plot types, where only a superficial connection between the first and the next game via the outro existed. The Witcher 1 had a "microplot" that basically took place in Vizima with a small detour and barely evolved politics, where The Witcher 2 had a "macroplot" on a grand scale that moved from one country to another.
Playing TW2 without TW1 didnt mean a lot apart from optional stuff like lore entries, which by no means was necessary to enjoy the second game.
Hence I think it will be a similar experience from TW2 to TW3. Its not a big overarching story like the ME one where you have:
ME 1: Discovery
ME 2: Interlude
ME 3: Confrontation
They were clearly connected.