I agree with others that you should make sure that you really do think this is an issue. But if your kids really don't listen when you tell them to do (or not do) things, if you feel like you are always repeating yourself or saying "no," or if you just don't bother saying "no" half the time because you know it won't work, then you probably do want to take some action. Time outs might not work for the 8-year-old, but they probably will for the younger ones if you are consistent about them. I've found that in order for time outs to be effective, first of all they must be short (a couple of minutes for the 2-year-old and 4 or 5 minutes for the 5-year-old; we use a digital timer so that the length of the time out is not in our hands and can't be negotiated during the time out). Next (and this one is particularly hard for me), I must give a time out at the FIRST transgression. No second chances, or my kids quickly learn that they don't need to do what I say immediately. So if I say not to turn the faucet on and my son turns the faucet on, I don't say, "Do that again and you get a time out." I immediately put him in time out, even though he's saying, "I'll stop doing that now. I'm listening now." It's no fun to put my kid in time out, so I'm always tempted to believe his promises of good behaviour and give him a second chance. But even if he does listen that time, it only encourages him not to listen the next time, and ultimately my life gets more frustrating.
For the 8-year-old, you might need to find a different consequence. Just make sure it is something that will really be a punishment. You could take away a toy or privilege except that once you've taken it away, you have nothing left to take away if he has a second transgression. My kids are both younger than that, so I'm afraid I don't have any ingenious suggestions.