Did you just look at champs that start with L?
Kidding aside, Leblanc and Lux can one shot squishy targets with a fairly minimal AP investment. Lissandra was designed specifically with longer engages in mind and for close range kiting. She can one shot people, but it takes a little more AP.
Consider the following list of total damage per single combo at level 6 (note this assumes Leblanc does not mimic Distortion; Lux hits with 1st part of Q and procs passive once with Final Spark):
Ability Ranks: Q:1 W:3 E:1 R:1
555 Base damage + 360% AP = Total Damage
Ability Ranks: Q:1 W:1 E:3 R:1
613 Base Damage + 225% AP = Total Damage
Ability Ranks: Q:3 W:1 E:1 R:1
420 Base Damage + 235% AP = Total Damage
Leblanc starts dealing more damage than Lux when both have >44 AP. Not hard to do, but Lux plays safer and offers more AOE, CC, and Utility with picks and shields.
Lissandra does more damage than Lux when both have > 1930 AP. Not gonna happen. So is Lissandra a bad 1 shot champ? Not necessarily. Remember what I said about close range kiting? At rank 5 with 0% CDR, Lissandra's Q has a 3 second CD. With a rank 5 W and an R, the target is chain CC'd in place for up to 3 seconds. SOOOOOOO . . . lets see how the ladies stand @ Level 18
@ level 18, AP ratios are the same but Base Damages are:
: 1,435 BD + 360% AP = TD
: 1,154 BD + 225% AP = TD
: 1,020 BD + 235% AP = TD
(with a 2nd Q): 1,210 BD + 300% AP = TD
With that 2nd Q, Lissandra's "combo" out damages Lux. It just doesn't happen in 0.5 second.
So.
Lux has Utility and Pick potential Like Morgana.
Leblanc has high Base damage and AP scaling Like Morgana*.
Lissandra has Team Lockdown and survivability like Morgana.
All three are good and can burst, It just depends primarily on what your team needs from you (lane matchup is secondary)
*
@ lvl 18 (assumes 3 second Q snare and 1.5 second R snare are sufficient to keep target in W for 5 seconds):
1500 BD + 395%AP = TD
0.o Riotnerfplz
jk ; )