Paramount’s “Paranormal Activity 3″ scared up good business at the domestic B.O. last night, grossing $26.2 million.Though most insiders called “Paranormal’s” triumph early, its margin of victory is already a little surprising. Stateside B.O. has been disappointing lately, especially in the under-25 crowd, but Par’s third installment is already posting a 30% increase over “Paranormal Activity 2″ and 53% of its aud was younger than 25. At its current rate, “Paranormal” could land just north of $50 million this weekend, with promising legs as October won’t see a new “Saw” release to distract the marketplace. In distant second, Par holdover “Footloose” made $3.5 million last night. After winning Friday, pic took a minor beating last weekend from Disney’s “Real Steel,” which grossed $3.1 million on Friday for third, and this frame may be no different depending on the apetite of auds too young for “Paranormal.” Newcomer “The Three Musketeers” opened to a modest $2.9 million last night. Though just yesterday observers had the film pegged around $11 million, Summit is adjusting its expectations to the $8 million to $9 million range. Universal’s domestic bow for “Johnny English Reborn” made $1.1 million last night. The Rowan Atkinson laffer has already done superb business abroad, but isn’t expected to repeat its success Stateside.