You'll want to play the other games eventually, but you don't need them to understand the third game as each installment is meant to be it's own game. However, you'll experience certain spoilers that are much more enjoyable to see unfold during the progression of all three games, but still, I remind you it's your choice to decide if you can live with that. And as for the earlier mythos, it's definitely an extra. If you want the full .hack experience though, you'd need to watch .hack//sign, play the first four .hack games, watch .hack//legends, read .hack//roots, play .hack//GU, and watch .hack//GU roots. Uhuh. Uhuh. Uhuh-uhuh-uhuh.