In the third paragraph of Shirley Jackson's story "The Lottery," what suggests that the lottery is a serious event?

Because the mood of the narrative changes in the third paragraph, the reader is alerted to the seriousness of the townspeople's gathering for the lottery.

In the first paragraph, there is a peaceful mood created by the description of a seemingly perfect summer day:

The morning of June 27th was clear and sunny, with the fresh warmth of a full-summer day; the flowers were blossoming profusely and the grass was richly green.

In the second paragraph,...

Because the mood of the narrative changes in the third paragraph, the reader is alerted to the seriousness of the townspeople's gathering for the lottery.

In the first paragraph, there is a peaceful mood created by the description of a seemingly perfect summer day:

The morning of June 27th was clear and sunny, with the fresh warmth of a full-summer day; the flowers were blossoming profusely and the grass was richly green.

In the second paragraph, the mood also seems relaxed as the behavior of the children and their talk of school that has just let out for the summer seems natural. The girls who are a little older converse as they glance over their shoulders at the boys in a casual manner. The small children roll delightedly in the dust or hold onto the hands of siblings. 

In the third paragraph, as the men start to gather, they make a quick count of their children, but they soon settle into speaking of the usual topics of farmers: planting, tractors, and taxes. However, there seems to be some nervousness in the men as they almost huddle together away from a sinister pile of stones in a corner near them. Also, that the occasion of the lottery is an affair of grave consequence is indicated as "their jokes were quiet, and they smiled rather than laughed." Finally, when the wives appear, they gather the children to them in what seems a protective manner. This action, too, suggests the gravity of the occasion. Certainly, the tone of this third paragraph is more somber than that of the first two, suggesting that the occasion of the townspeople's gathering for the lottery is a serious event.