So, with interrogative pronouns, do we use verb in its 3rd person singular form? Or should be still need a hint for the question? For example, could we say "Qui parlent?" when we heard several different voices?
No. It is a common misconception when starting out but when speaking French, you must think in French and not think "who is speaking" in English and then translate it word-for-word to French. The result will often be wrong. The French "qui parle" may be translated as "who speaks" (simple present tense) or "who is speaking" (present continuous tense). There is no separate word to put "is" there.