Of the three, the True140 is the best of the bunch (especially at this week's offer price of £29.99).
True 120 has 4 heatpipes
True 140 has 6 heatpipes
TX3 has 3 heatpipes and uses a 92mm fan. Not a bad cooler, but you can do better at that price point.
Apart from the extra couple of heatpipes on the TR140, you're also getting a great fan that also graces Thermalright's top dog Silver Arrow model which is regarded as the quietest top end cooler on the market using one of those fans plus a 150mm version.
If you can afford it, it's the one to go for.
The 120 is a decent choice too and there are other alternatives in the segment.