Me personally, I'm about 70 hours in the Witcher 3 but so far it seems that TW3 has more to do with the books than with what happens in TW2. I might be wrong though, I haven't played all that much main story since I arrived in Skellige soo.....
Witcher 2 was unbelievably good though. I really, really loved the character progression and skill development all through the game. Damn, the beginning is tough and unforgiving. 1st time I went through as a sword fighter on Roche's path, 2nd time as a Signs build on Iorveth's path years later in prep for TW3 and the 2nd time I had a LOT more fun. I think the Sign build was a bit overpowered by the end though.
In short, I wouldn't bother finishing TW2 first, it's not crucial as TW3 is such a HUGE game in it's own right. Yeah, it's good to know who is Triss and Zoltan and Dandelion but that all becomes pretty apparent. Dandelion is the freaking narrator and pretty much codex author of TW3 anyways. I can sum up what you need to know about Triss and Geralt's history in 2 sentences from TW2 (and Yen from the books) but that's a spoiler I wouldn't write here.
Just know that they're very different (TW2 linear, TW3 open world obviously) but both fantastic gaming experiences - TW1 and TW2 just take a lil longer to pick up the pace.