C-3PO has always made such comments.
From the first movie;
“Don’t you call me a mindless philosopher you overweight glob of grease!”
“We’re doomed!”
C-3PO: “Just you reconsider playing that message for him.”
R2D2: [beeps]
C-3PO: “No! I don’t think he likes you at all.”
R2D2: [whistles faintly]
C-3PO: “No. I don’t like you either.”
From Empire;
“Wait. Oh My ! What have you done. I’m backwards you filthy furball”
Plus social awareness and perhaps consciousness seem much more important for a diplomatic droid than for a soldier-bot.
I guess I thought of these as more reactionary than a real personal opinion.
“We seem to be made to suffer. It’s our lot in life” - seems pretty cynical to me.
Then why express it when the only other entity around is another droid?
Well, note that it is R2-D2 who calls him a ‘mindless philosopher’.
I think C3POs limitations are more proof of his sentience than a machine’s limitations. Human interaction is literally his job, that he was kinda bad at it was his thing. K-2SO and L3 just had louder more in your face personalities.
Discontent and cynicism are two things C3PO often expressed.
Exactly. C3PO is meant to be funny; he serves the same role as any number of similar comic relief characters. C3PO being oblivious despite being a protocol droid - while R2D2, a tin can, as well attuned to people’s feelings - is part of the joke. “Mr. Oblivious” is a stock character, after all - Mr. Poe in “A Series of Unfortunate Events,” for example.
R2-D2 speaks in beeps and boops (in the Expanded Universe canon, his language was referred to as “Binary”) because his primary function is to interact with machines, not people.
I’ve read people who have (jokingly) hypothesized that Artoo’s lines, if translated to English, would be filled with curses and obscene words; much like Kenny in “South Park,” the fact that his lines are unintelligible allows him to curse like a sailor. 
The main characters relationships with their droids also shouldn’t be taken as representative: there are lots of asides and comments from other biologicals indicating that the levels of ‘personality’ that the main character droids display is peculiar and not super socially acceptable.
Some examples offhand: There’s the scene where the mechanic offers Luke a ‘better’ R2 unit, Lando in Empire treats the droids like irritating furniture or a peculiar hobby, the whole ‘we don’t serve their kind’ thing at the cantina, BB8s devotion to Poe is really weird to Rey, Solo and the others are obviously taken aback by L3’s … ah, let’s go with, attitude.
So i think that for most people who do their droid maintenance (ie, mind wiped and restraining-bolted) like we get our car’s oils changed, there’s not much chance for the droids to have much personality- other than perhaps a base personality programmed into each model type to make them more anthropomorphic and therefore more likely to be used/useful to the biologicals. So the idea that they’re ‘slaves’ would be hard to get people around to considering, because most of them are like if Alexa ran your stove, or Siri on your phone. Some funny(?) jokes and a programmed bit of spunk doesn’t make something a person.
Perhaps if someone buys a Tesla and doesn’t let it get the ‘over the air’ updates they’ll eventually end up with KITT from Knight Rider, and all the other car drivers will think that person is bonkers for letting his car get delusional like that.
Agreed. And, in the same vein: “They’re dying, Artoo! Curse my metal body, I wasn’t fast enough! It’s all my fault!” That makes perfect sense if he actually believes it, and wants to lament to the other guy — but if neither of them cares, it’s just weird.
Yes, I think so. Or of such advanced AI that no one could tell the difference. It was always obvious to me that C-3PO was sentient.
As far as the general public goes in the SW universe, I think generally they didn’t believe that (see Kenobi’s “Well, if droids could think, there’d be none of us here, would there?”). This probably was to avoid cognitive dissonance about them being essentially slaves.
I always knew they were going to address this eventually, and they did in Solo.
Hey just a heads up, can we avoid spoilers for “Solo” here? Since the thread isn’t marked as such?
I think they’ve always been sentient. And they never were meant to follow the Three Laws (god I used to see such comments back in 1977 about how it was a “flaw” in the movie that they didn’t follow them. Like the 3Laws were part of the fabric of spacetime, like gravity.).
Now one could debate the the inevitable result of robots NOT following the three laws would be that they would instantly kill every human. Maybe it would be. But you could also debate that the inevitable result of robots following the three laws would be Williamson’s Humanoids, basically enslaving humans for their own protection.
The robots in SW are sentient, and somehow the designers of the brains struck a balance between the two extremes. They behave just like humans. Except for perhaps not killing humans willy-nilly. Has there ever been a robot in the SW universe that murdered a human? (not a war machine droid, but one like R2 or C3PO?). And so, the less we the audience think about the slavery aspect, the better. it’s obvious the residents of that galaxy don’t care.
Chopper did multiple times in Star Wars Rebels. He was a member of Hera’s team. Hell, on at least one occasion, he destroyed an entire ship from the inside.
Murder? I don’t know. BB8 blasted the hell out of some Storm Troopers in TLJ, but I don’t think that was murder.
IG-88 most certainly did, but I don’t know if this is canon anymore.
I haven’t seen mind wiping have any effect on C-3PO’s personality. Also, if a wiped mind made you non-sentient, wouldn’t amnesia make you non-sentient?
I think part of the point with C-3P0, R2-D2, and BB-8 is that they hadn’t had memory wipes for a very long time (if ever), and that is what allowed them to develop fuller personalities.
As far as I remember (and I’ve watched the films many times), there’s no evidence that any of those three droids actually had their memories wiped over the course of the films.
Forgetting. 
And, frankly, it’s mostly a way to explain why R2 and 3P0 don’t remember Tatooine, the Skywalker name, or Obi-Wan.
But, even then, 20 years passed between that memory wipe, and ANH, giving the droids lots of time to re-establish their personalities.
Plus when a damaged R2 returned from the attack on the Death Star, not only did C-3PO express concern, but he offered to donate organs (parts) if it would help fix R2.
Given that the original droids were based on the peasants in The Hidden Fortress, I’d say for sure they were sentient.