grammar - Is 「３人いるだと」 grammatical?

３人いるだと！？

Can you say ３人いるだ as a statement? If だと is simply だ + と, I would expect ３人いるんだと.　Is だと considered a particle in its own right, with different rules than だ?

Transcription of the contents of the picture:

A: [何]{なに}がどうなってる!!
    17[区]{く}にも[傷の男]{スカー}!?
     じゃあ　3区のはなんだ!?
B: ダグラス大佐！
     8区にも出ました！
C: ……3人いるだと!?

Answer

だと！？ at the end of a sentence, e.g. 何だと！？ "say what!?" can indicate shock or disbelief, e.g. "you say there are three people there!?"

Edit: The だと seems to have come from だって. According to Daijisen's last definition:

［終助］《係助詞「だって」の文末用法から》引用句に付く。相手の言葉に対して、非難・驚きの気持ちを込めて強調する意を表す。

[final auxiliary] 《from the binding particle だって's end-of-sentence rules of use》 Attaches to a quotation. Indicates additional emphasis of criticism or surprise in regards to the other person's speech/words.