This question comes across the internet pretty regularly, in blogs, discussion boards, listservs, you name it. Though I've been seeing it more often lately. Not sure why.
The short answer to this question is this: you MUST finish your novel first. Why? For many reasons.
1. Not everyone is capable of finishing what they start. Since agents and editors don't know you personally, they don't know if you possess that particular skill. So if you approach an agent with only three chapters and an outline, they won't know if you will finish the first draft, plus get through all the necessary rounds of revision (once an editor buys your work, you will have to go through at least one round of revision - it's just the way it is).
2. It takes many drafts to make a good novel. Whether you revise as you go or write multiple drafts, you still need to re-read what you've written. If you don't, then you really have no idea what kind of quality you're submitting. It also means you're sending out a project that does not reflect your full potential. Therefore, you're not showing the agent or editor what you can really do. It makes it easier to set your work down.
3. As you write, you will likely make discoveries about your characters, setting, plot, etc, which could change the way your story unfolds - even if you've planned everything out. In other words, the agent/editor might end up with a different product than she expected. That could raise some issues, such as cancelled contracts.
4. These days, the query count has skyrocketed. That means your work really needs to stand out from the massive pile on the agent/editor'S desk. An incomplete project isn't going to do this, because it is not the best that it can be. 'Potentially good' isn't going to cut it, because there is still no guarantee that you will finish what you started. So, it's really in your own best interest to send out a novel that you have gone through more than once, and make it as good as you possibly can.
There are exceptions, of course. If you have already published a novel (not self-published) and proven that you can make it through the process, then your subsequent books will likely sell on proposal. Also, non-fiction generally sells on proposal - three chapters plus outline. But fiction doesn't work this way for debut authors. We get to work on spec until we establish ourselves.
Many writers start querying too soon. Either they don't want to put in a bunch of work for a project that might not sell, or they are too excited about the story to wait until it's done. But we get one shot with agents and editors, so we need to make sure to hit them with the absolute best work possible do they will have no reason to say no. An unfinished project will never do that.