Three people have already arrived, but there are still two more to come.
「3名来てます！あと2名まだです！」と言うなら次のように言うと自然な感じで良いでしょう。
ーThree people have already arrived, but there are still two more to come.
「３名はもう来ましたが、まだあと２名来ます」
「A君とB君は来てるけど、C君はまだです」と言う場合は、not yetやmissing を使って言えますよ。
ーA and B are here, but C is not here yet.
ーA and B are here, but C is missing.
ご参考まで！
カナダ 
