这3个动词均有“哭”之意。

cry : 普通用词。指因痛苦、悲哀或伤感等出声地哭。

The baby can cry as soon as he is born.（婴儿生下来就会哭 。）

weep : 书面用词，指小哭或无声地哭，侧重流泪。

She wept her sad fate.（她为她的悲惨命运而哭泣。）

sob : 指抽泣、呜咽。

The child started to sob when he couldn't find his mother.（孩子因找不到他妈妈哭了起来。）