"It" is functioning as an empty subject in all three of your examples, and "it" is always singular. The words after "was" in all three are serving as predicate nominatives. The verb always needs to agree with the subject, not the predicate nominative.

It’s correct. It is strange though now you mention it. “Those” is plural

All three sentences are correct. Consider these sentences: What was it that you really liked? ("What" is singular but doesn't require that the answer be singular) I really liked these subjects. (no problem here. you can think of "these subjects" as a plural or as the single thing that is liked, a collection (singular noun) of subjects) I really liked these. (same) It was these that I liked. (same sentence, really) It was these subjects that I liked. ("subjects" just adds a bit of clarification)

Thanks for your prompt responce! You've helped me a lot. Now I can sleep well :)

Looks rite to me. Sounds rigid when spoken out loud but you got it.