It was hauling, a noted Moderator Emerita, Community Leader and Epic Contributor, who once wrote that "in my experience, the first inq on EQ is "free", meaning no score drop. The second costs 3-5 points. Then the third one is free, and the fourth costs a few more points."

OP, I'm not sure what you mean by "only the INQ which stands at 3", but if your number of inquiries from within the last year is 1 or 3, then that first or that third inquiry may not decrease your score, in which case of course your drop is a mystery.