Short answer - You will want to install both CT's on each hot, and make sure the unit is set to 120V (looks like the default for #1 and #2).
Long answer - The sensors are essentially clamp on transformer secondary coils. The varying magnetic field around the cable induces a current in the coil. The unit measures this induced current to calculate the current flow through the wire. Multiply that by the voltage carried in the line over time and you have kWh.
These systems are intended to be installed at the main panel, with a CT on each incoming 120V line, and the voltage setting in the monitor set to 120V. It then measures each leg separately, since the 120V loads in your house are unbalanced.
I scanned through the Efergy manual. It looks like you can set it to 240V, then use one CT on one hot of your 14-50. Assuming no ground fault, the current on the two legs will be equal. You could then use the second CT on some other 240V load, but it looks like the display will only show the combined usage of both, so I don't see much advantage in this setup. Plus, they'd have to be plugged into the same transmitter, so the two sensors need to be near each other.
#1 looks to be settable to 240V, so I guess you could use 1 CT, but the ebay description is not clear (not a native English speaker)
#2 can bet set to 240V, and has a link to a full manual (see page 20 - can run 120/240 split phase, up to 575 3-phase)
#3 has no useful information in the description. But it does have an utterly useless CO2 (not CO) detector built in, according to the video.