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!?
だって?'/'ですって?', so I think the sentence here can be rephrased as 「『3人いる』だって?」/「『3人いる』ですって?」-- The difference is that「『3人いる』だと?」sounds masculine. (Can this be any help?) – May 30 '12 at 22:45何だって?/何ですって?/何だと!/えーっ!. Would you say that何だと!?is masculine though? I think I've seen girls saying it on online forums. – cypher May 30 '12 at 23:09なんと!which might be different again. – cypher May 31 '12 at 08:05「3人いる」。だと言った!?considered grammatical? If so, is it possible that3人いるだと!?is a shortened version of it? – Pacerier May 31 '12 at 18:39