the auxiliary “need” (in affirmative sentences?)

I remember being told the modal “need” is used only in interrogative and negative sentences and was for quite a long time more idiomatic than the normal forms, but is there anything wrong with the following examples, in which “need” can seemingly be replaced with “must”?

1. All you need do is ask.
2. He wonders if he need go.
3. In order to pass the course you need pass the exam.

Is the only crime here some awkwardness, or are these ungrammatical?

Thank you!