It depends on the size of the celery sticks. For example, there are:
Approximately 6 calories in 3 small stalks of celery (about 5" each)
Approximately 18 calories in 3 medium stalks (about 7"...
That depends on the size of the celery stick (small, medium or large). For the calories in celery sticks according to size, see the page link, further down this page, listed under Related Questions.