Is there any grammar case in which the verb to be usually used for plurals "were" can be used for He/she/it?

Yes it is possible in Conditional sentences eg. If she were here, she would tell me what to do.

If I were you I would return the money, If it were any other day I would help you.

This used to be the rule about conditional sentences but now it is acceptable (and probably makes more sense) to use 'was' with the third person singular i.e he she it and also I. I know, English is confusing right? Try TEACHING it. :)

It is possible to use "were" for the singular pronouns (I, you, he, she, it, etc.). In the subjunctive verb mood, the past tense of "to be" verbs is, in fact, "were." When you speak in "what ifs," such as "If I were a bird..." or "If she were cuter..." it's grammatically correct.

Naw proper grammer for "He were..." would be "He was about to....." Were is a plural past tense (aka they were, they are, they are going to)