Whether 3a is grammatical is dubious; it certainly is not idiomatic. 3 is grammatical and idiomatic.
It is hard to explain why 3a is not good English. I believe the most obvious explanation relates to the "it is" construction. As you know, a valid English sentence must have an explicit subject. For various reasons, it may be hard or inconvenient to find an explicit subject. In that case, we have three constructions that act as dummy subjects, namely "it is," "there is," and "there are," where "it" or "there" act grammatically as subjects without any concrete referent.
It is a weird feeling
is such a sentence. "It" refers to nothing concrete except possibly itself, and the sentence
A weird feeling is a weird feeling
is true, but hardly informative.
When you use the apposition, the "it" suddenly has a referent.
It is a weird feeling, wearing her jacket
Wearing her jacket feels weird.
The intended meaning of the first translates into a sentence where "it" is replaced by the gerund "wearing."
"Which is" does not directly refer back to "it" and so seems to be missing the point, namely to clarify what "it" refers to.
This explanation of why 3a is not idiomatic is a rationalization of the fact that 3a is not idiomatic. Whether the rationalization is correct or not does not alter the fact that 3a is not idiomatic.
This leads me to a point of style. "It is" and "there are" are very common contructions, but I try to avoid them in written English and use instead a concrete subject.