Spokes loose on a new wheel, is this expected or a bad build? So a couple of months ago, I bought a new front wheel from the local bike shop. It's a 28 spoke wheel, rims are the DT Swiss RR465 and the hub is a 240s.

I put 500-600 km on it and noticed a rattle after a ride. After isolating it to the wheel, I noticed that 3 of the spokes were extremely loose with one of them basically hanging. (The nipple/screw thing appears to be inside the rim somewhere.

Anyhow, I'm pretty sure I didn't hit anything and I'm not a terribly large guy. Are there things that can happen to new wheels/spokes, stretching or coming loose? Or is this a case of the LBS basically doing a crap job of building the wheel?

I'm nearly positive I didn't do anything wrong here, but I'd like to know if there was any possible user error.
Apart from an impact, if it's been twisted in a bike stand wheel slot or something that could cause loose spokes. But usually loose spokes on a near-new wheel is just a bad build - if the spokes haven't been stress relieved properly they'll generally untwist as you ride it. And some wheelbuilders don't use a spoke tensiometer so regularly produce wheels that loosen over time.

Either way, take it back to the shop and ask them to fix it. When you collect it after they fix it, pay very close attention the very first time you put weight on the wheel. The first revolution with load on it is crucial - if you hear pinging noises that is the spokes untwisting themselves as the tension goes off them (the ones at the bottom have less tension due to your weight pushing down, so there's less resistance to them untwisting). That means the wheel hadn't been stress relieved properly (because you just did it), so the spoke tension might not be correct any more (and the wheel might no longer be quite true). If the problem happens again take it to someone different - someone who can't fix a wheel they built is not worth your time.

In more detail: as a wheel is built or trued each spoke nipple is turned. Because there's friction between the nipple and the spoke, this tends to twist the spoke as well as tightening or loosening the thread. Stress relieving means (gently) applying force to the wheel to momentarily loosen each spoke, allowing them to spring back into the non-twisted position. This slightly reduces the spoke tension, and may even unscrew the spoke slightly out of the nipple causing further loss of tension. So after stress relieving a good wheelbuilder will recheck the spoke tension. 

EDIT by an anonymous user: Bladed spokes shouldn't twist because as they are built they are held with a tool which does not allow that.

A good wheel is not just true, it has even spoke tension. Over time uneven spoke tension tends to pull the wheel out of true, and the tight spokes are more likely to break in an impact, while the loose ones are more likely to break from metal fatigue. There's much discussion of this online, and Jobst Brandt's "The Bicycle Wheel" is a deceptively concise but detailed explanation of a lot of the issues. It's worth buying a copy if you're interested in wheelbuilding (I don't get anything out of you buying the book, I just think it's an excellent book).