He didn't work hard and as result of that he failed.
He didn't work hard,as a result of which he failed.
He didn't work hard,because of which he failed.
第二第三句（有which出现的）是定语从句,修饰代替的就是前面分句He didn't work hard整个句子,所以中间用逗号隔开,主句是逗号前面的,后面的是从句.
而第一句话呢,and是并列连词,连接2个同等成分的句子,没有从句.that在这里是代词,代替的就是前面那句话的意思.