Those aren't good examples of cowardice because these weren't situations of liesurely martial arts sparring or anything like that. Instance 1 was a matter of trying to avert a war between nations. Instance 2 was an instance of trying to save the entire world. Instance 3 was an instance of Sakura trying to save her spouse and child.Is coward thing to do.
In any martial arts fight or rather any fight attacking from behind is a cowards sign.
In Naruto the only coward i have seen is sakura.
1.She tried to kill sasuke from behind(failed tooooo hard).
2.She wasnt even in the kaguya fight but attacked kaguya when she was heavily distracted by naruto and sasuke.
3.She even hit Shin when he was distracted by the duo.
Not even tsunade does these things, she went all out against madara and head on did the fight honorably, hell when rock lee was almost beaten to death but he preferred attacking garra head on.
Do you guys know other cowards in naruto except sakura?
In instances like these , you do whatever is necessary. These aren't sporting events where pride and honor are involved.
These were real life and death situations.
If a burglar was in your house and he was threatening to kill your family , are you telling me that you'd avoid using underhanded tactics to take him down and that you'd risk letting your family die just for the sake of fighting honorably?
I'd hope not. It is not cowardice to do stuff like attacking from behind in serious matters. It is only cowardly if you do it during stuff like arranged sporting events or street fights that are simply for pride and honor.