The pair of 140's probably is excessive. Assuming you set the mobo CHA_FAN or SYS_FAN headers to do their default automatic fan control based on the temperature measured by a sensor on the mobo, all of those three case ventilation fans will receive the same signals (PWM) to set their speeds as percentages of full speed. So if all fans are identical, the two front intakes could pull in EACH as much air as the ONE rear exhaust fan can move out. Even allowing for the small reduction in intake fan capacity due to the intake air flow path and the front dust filters, the two intakes combined still will have more capacity than the single exhaust. This combination means the actual air flow rate at any particular fan speed setting would be somewhat more than one single unconstrained fan can do, and the case internal pressure will be slightly above ambient to reduce dust intake at leak points.
Now, change that scenario to having the front fans in the larger 140 mm size. For the same PWM signal as before, such a fan will run slower RPM's but will likely produce a higher air flow rate. Since the 120's would already have supplied more than the rear exhaust fan can do, this new even higher intake rate is pretty much wasted - it only will change very slightly the overall air flow rate and internal pressure. It might very well mean slightly LESS noise generated by the front fans because they are running more slowly. But in terms of cooling capacity, the difference would be small.
IF you anticipate that you will be operate often at maximum workload and high temperatures, having a slightly higher maximum air flow rate for slightly more cooling under those conditions would make the 140's an advantage. Otherwise the 120's will do the job well.