With the current difficulty of 609,482,679 you could generate in theory with 3TH/s about 0.1031 BTC/Hour or 2.474BTC per day.
I use this website for example:
http://bitcoinwisdom.com/bitcoin/difficultyIf calculate for delivery time in march next year, the average expected difficulty could been 5,941,344,136
Depend on this difficulty, the amount will decrease extremly:
0.01057BTC/Hour and 0.2537BTC per Day.
by March, BTC is worth around $1,500. It means around $350/day (based on 0.25BTC/day rate). $350/day x 30 days = over $11K.
If that is true, I would buy 100 3TH/s miners in a beat.