Do I need to join a pool if I have a 3TH mining rig? I am thinking about purchasing a 3TH mining rig and would like to know if it is still worthwile me joining a mining pool to mine BitCoins?

Do I have enough power to go it alone?
It is hard to decide right now, knowing that till you will buy, and the company will deliver your hardware, the difficulty will change many times.

Right now with a current difficulty, solo mining with 3Th/s will give an estimation of mining a block in 2 weeks (during this two weeks the difficulty may change). So even right now I would rather have a steady 2 BTC per day then to risk. Especially if after buying this hardware, you need to break even as fast as possible.