It's a phase. A horrible, horrible phase called childhood. :)~
No, really-- let me ask you-- what happens when they complain? What is your response. Do you discuss it with them? Reason with them? Or do you just give them a couple of blinks, shrug your shoulders and go about your business?
What I am asking is simply: how much attention are you giving it? If you scold, or reason with them, or give them any real attention, you will likely be setting yourself up for more complaining.
When my son complains about his food, I just say "this is what's for dinner. Eat it or don't. You are the one that will be hungry later on." I don't point out to him that he liked dinner last night or the amount of time I spent on it or anything like that.
Sometimes, if he were to say "Moooooom! There's not enough syrup on my pancake!" I would give a correction in a calm voice. "Okay, you've told me something. Now, do you have a QUESTION for me?" This is his cue to *ask* instead of complaining. You can have more syrup on your pancake mister, you just must trouble yourself to ASK for it because I am not a mind reader.
My sister also cues her complainers with a 'try that again' if it's something that can be a question.
Bathtimes, handwashing, etc.-- I just ignore the complaining. Outright stalling :"If you can't do the job I ask, you may sit in this chair until you are ready. "
Three and six are also capable of making themselves peanut butter and jelly sandwiches if their noses are bent out of shape about what's being served. Suffice it to say, my son knows how to make a sandwich. He's five, but has been doing it since he was three. :) Overall, though, unless it's a problem, I ignore the general complaining.
Unless, of course, we're stuck in the car and am hearing nonstop griping. Then it gets to a point where he has to pay me a nickel for every time he gripes instead of asking for what he needs. There's an idea: Give them a bag of nickels at the beginning of the day, say 50 cents worth each, and tell the boys they can keep whatever they don't spend on complaining; at the end of the day they can put the nickels they haven't used in their piggy banks. Might be a novel way for them to notice the complaining themselves!