Way

#2 is wrong.
#1 and #3 mean the same, but #1 is the better version.

Are you sure that #2 is wrong?

Plus, if #3 is correct, the relative pronoun is an objective case, and in #1 the objective relative pronoun is omitted.

But what about this one, then?

They accomplished it (in) the way they wanted to.

Wouldn't it also work?

Beyond the grammar issues, the problem with that text is the mismatch in formality.

"Accomplish" is invariably used in formal contexts, whereas "the way they wanted" is not very formal.
What exactly is it that you're trying to say, and how formal do you want your sentence to be?

TakaDo they all work and mean the same thing?They accomplished it in the way they wanted.They accomplished it in the way in which they wanted.They accomplished it in the way which they wanted.

The only 'problem' I see is that I would naturally add 'to' at the end of each sentence. To me, the third sounds a little strange with 'which' instead of 'that'.

Mister MicawberTo me, the third sounds a little strange with 'which' instead of 'that'.

Same here.

So it seems safe to conclude that the first one is not one of those in which the objective relative pronouns are omitted.

I would naturally add 'to' at the end of each sentence.

It doesn't mean that without "to" they w

TakaIt doesn't mean that without "to" they would sound wrong, does it, MM?

I don't think so; they just sound 'not quite right' to me.

OK. So without "to", they are still acceptable. Good.

Thanks, MM!

(And thanks for the comments, teechr!)

I'm so sorry for my interfering in this thread.

1. They accomplished it (in) the way (that) they wanted.

park sang joonI don't know why #3 is right, Could you tell me the explanation of why #3 is right.

'which' = 'that'