As I know, several theories exist. Yeah, it was quite old issue, so it's not that much problem in these days.
1. Modern theologians insist that it's majestic plural. Yeah, Hebrew language has it too. There's several more examples in bible.
2. Some modern theologians insist that it's a reflexive pronoun.
3. Philo (20 BC – 50 AD) insisted that those plurals mean god and angels, but this theory isn't accepted that widely in modern Christianity.
4. Classical theory about this pleural is, those pleurals mean the Father, the Son and the Holy Spirit.
Several other theories exist. Don't ask about them to me, I'm no Christian.
Too serious reply in off-topic, lol.