What is the ruling on attending circles of admonition given by older students who are not scholars, and is turning away from them sinful, like the Hadith of the Three People?

It is permissible to attend sermons given by someone who is not a scholar, as long as it is highly probable that he only speaks what he has learned from scholars and does not issue fatwas without knowledge. This applies even if he limits himself to reciting authentic hadiths. He must be mindful of Allah in what he says and only speak what has been previously stated by reputable scholars.

As for someone who avoids gatherings of remembrance out of fear for his religion due to a lack of trust in the speaker or for another valid reason, there is no blame upon him, Allah willing. The hadith mentioned, however, speaks of turning away from the Prophet's (peace be upon him) assembly without a valid excuse.