YOU SAID:

I have discovered that my sister has three horses.

INTO JAPANESE

私は私の姉が3頭の馬を飼っていることを発見しました。

BACK INTO ENGLISH

I discovered that my sister has three horses.

INTO JAPANESE

私の妹は3頭の馬を飼っていることを発見した。

BACK INTO ENGLISH

My sister discovered he had three horses.

INTO JAPANESE

私の妹は彼が3頭の馬を飼っているのを発見した。

BACK INTO ENGLISH

My sister discovered he had three horses.