When the Kennedy Center Opera House box office opens at 10 a.m. Tuesday for seats to "A Chorus Line" there definitely will be some tickets for sale.

At least 3,000 tickets in all parts of the house for most performances have been returned to the center by the Theatre Guild, its subscription sales having been smaller than anticipated.

The award-winning musical previews Sept. 14. opening Sept. 15 at 8 p.m. There is a preview matinee on the 15th at 2 p.m. for which tickets are still available. The Washington Chapter of the Juvenile Diabetes Foundation will benefit from the preview for which some seats remain. Phone: 434-7451.

An augmented box-office staff is spending the weekend trying to track down mail orders returned before the Theatre Guild released a portion of its subscription tickets. Some of those returned orders were for performances and locations held by the Guild. The senders are known to the center.

Higher than in privious years Guild subscription prices - for 10 productions scaled from $76.25 at midweek matinees to $141.75 for Friday and Saturday nights - are thought to have been a factor in cutting down the Guild's expected total.