The choice that sounds natural to me is the 3rd.
On this occasion my class prepared a celebration in which I was a host.
Because "celebration" needs an article. so the first choice is out. It cannot be preceded by "the" unless it's mentioned in a previous sentence. So the 3rd usage sounds more natural given the context available.
Also I would like to weigh in on the discussion about Celebration vs party: I would substitute "celebration" with "function".
4 an official or formal ceremony or social gathering
<They went to several functions during their college reunion weekend.>
Final comment is why not
"of" which I was a host.