A PhD can be like a bad relationship--easy to get sucked into but difficult to end. Almost without realising it, you may find yourself drawn into the secret world of research and under the powerful spell of doing more and more experiments. But beware ... under this spell you can lose the power of rational thought, and as time goes by, you may even end up in a place that you can't, or won't let yourself, escape from. That place is called 'the laboratory of never-ending 'last' experiments.'
But a PhD is only valuable to you as the springboard to your future career. You need to finish it, and that means drawing your lab work to a close and writing the thing up. Now I don't want to appear melodramatic, but if you are well advanced in the third year of your PhD funding, you really do need to get out of the lab. I agree with the many people who've told me that writing up and lab work are mutually incompatible. Why do you think people are sometimes reluctant to offer postdoc positions to PhD students who've not completed their theses?
The effort of will required to escape from the lab is entirely down to you--don't wait around for someone else to tell you that it's time to make the break. In my experience, PhD advisors are always seeking just one more 'key' result from you. So don't expect them to pull the plug on your research at an early enough stage for you to submit within the 3 years. If you want a barometer of when you have enough results for your thesis, they may be the last person you should ask. Find someone else's advisor and ask them to be brutally honest with you. And whenever you do make the decision to stop work in the lab, stick to your guns. I had to say no to my boss several times before he got the message that I really wasn't going to do any more practical work.
Getting to the point of saying 'no' requires a strategy. First you'll need to write a wish list of final experiments. Your first draft of this list will probably be unachieveably long. It will also, almost certainly, take you even more time to complete than you've estimated. So, before ploughing into your list, first ask yourself 'is my experiment really necessary?' At least two of my essential final experiments, that somehow survived the initial cull, were eventually shelved. Looking back, I just didn't need them for my thesis. The point is that I really thought I did. I advise talking to good postdocs and reading their theses. This is a good way to avoid the delusion that you need to have finished absolutely everything on your list before you submit. After all, you have plenty of leeway to argue, both in writing and orally, why you didn't do something. Best of all is if you can portray the idea that you made a conscious decision to do another more informative experiment instead of the one you left out. Maybe, without even recognising it, that's really what you'll have done, and it's good science. It is even better to prioritise your final hit list so you can knock off outstanding experiments in order of their importance to your thesis. This way you always have the option of letting the stragglers drop off the end of your list if you run short of time, a bit like a newspaper editor chopping the end of a report one sentence at a time until it fits into the space on the page!
These final few weeks in the lab are the occasion to bring all your time management skills  to bear. Make use of any gaps in your busy schedule to prepare for writing. To start with, sort out the chaff from your files and back up all those floppies and CDs. In fact, do anything that might ease the pain of the impending write-up. Even if it's only the odd half-hour spent preparing one of your tables, it'll be worth it. When you come across these little tidbits of effort in the middle of your writing up, they'll give you a small, but probably much needed, boost. If you can't contain the urge to scribe, your best bet is to update, or start (tut-tut!), your Materials and Methods chapter. This stuff is fairly easy to get down and helps you realise just how much you've done.
So, now that you're raring to go with your pared down list of really crucial final experiments, crack on and get them sorted, once and for all. If you are already feeling run down after two and a half years slogging it out day-in, day-out with Mother Nature, you'll need to tap into all your reserves for this final push. And it's not as if you can look forward to a nice rest when it's all over. The whole point of the exercise is to get you writing up early and, often, that's a complete nightmare in itself. Perhaps an incentive is in order. I made plans to sneak off for a week's holiday between finishing in the lab and starting the big write-up. You'll never again find such a natural break point. (Or, after that last mad rush in the lab, do I mean breaking point?!) Planning a nice little trip away will give you something rewarding to aim for and, if travel tickets and accommodation bookings are involved, an unbreakable deadline.