3 CHILDREN FOUND SLAIN / 3 CHILDRED FOUND WERE KILLED

若這兩句都是標題則文化沒有不對（因標題會省卻be並它的變化形），若是完整句子，則第一句漏，were：全句是：3 Children were found slain.
除了文化外，這兩句其實意思略有不同

3 CHILDREN FOUND SLAIN：
是指：有三個小孩被發現被殺害

3 CHILDRED FOUND WERE KILLED
而在這句中found是修飾3children，可以看為3 children who were found...的簡化句，所以全句可譯做：
三個被發現的小孩子都被殺了。
希望幫到你！

2008-04-08 14:03:16 補充：
第一句的發現是：發現被殺害的事實
第二句的發現是：發現三個小孩子

3 CHILDREN FOUND SLAIN
slay
vt.
1. 【書】殺死,殺害
Two women were brutally slain last night.
兩名婦女昨夜被殘忍殺害。
2. 【美】【俚】(過去式和過去分詞常作slayed)使大為高興,使禁不住大笑
That clown just slays me.
那小丑真叫我笑死了。
vi.
1. 殺死;致死

意思相同應該是:

3 CHILDREN FOUND DEAD

3 CHILDREN WERE KILLED