Most people here would recommend getting the game, especially if it is on sale. I wouldn't worry too much about not playing the others. Of course being a sequel, there are going to be references you won't get regarding story and characters but the developers do a good job at reducing this problem. Otherwise, there are plenty of youtube videos explaining the story (I think ign do a good 5 minute one).
When it comes to combat, it's fine. It's not the best aspect of the game and certainly not as crisp as say Bloodborne's but is a million miles ahead of something like Skyrim. It takes a bit of time to get used to as at first it feels sort of clunky and repetitive, but as you get into the game, it starts to feel more fluid and more tactical.
Admittedly, it's not going to be for everyone. It's a bit of a slow starter, mixed in with initial awkward controls and potentially confusing story, can be a turn off. But if you can break through that, it's certainly best game of the generation by a long distance. For me, it's the best game I have ever played because of it's beautiful design, great graphics, brilliantly depicted characters (no game emits such a wit and dark humour as W3, it puts games like DA:I and ES to shame) and awesome amount of quests and content but without getting overwhelmed.
Besides, if it's cheap then you might as well go for it.