You didn't really mention what your upper limit was, but if the Enttec unit is too pricey, here's another option that's at least a little less expensive (about $200) -- the SAND Network Systems SANDbox:
http://sandsys.mybigcommerce.com/dmx-interface/?sort=priceasc
The cheapest model is about $200, and the box can be configured to speak ArtNet, SACN, etc. Can also be "flipped" to support DMX-to-Ethernet, which is handy when you're looking for an easy way to pipe DMX over existing CAT5 wiring in a facility, for example. I have a bunch of these things running the lighting in my studio (pics:
http://www.undisclosedlocationstudios.com/) and I really like them.
As for these units being 5-pin, if that's an issue for you, honestly I think you'd be better off just making yourself/buying yourself some 3-to-5 pin adapters rather than counting on finding a 3-pin version of these kind of units. I can't say that they don't exist, but in my experience, at least, the companies that make these sorts of devices are trying to get a bunch of standards (ArtNet, SACN, DMX, etc.) to play nice with each other, and that seems to push them in the direction of following the standards to the letter. Forgive my ignorance on this point -- I don't know that there's ever been an "official" 3-pin standard for DMX, only that many manufacturers (Chauvet comes to front of mind) have casually offered DMX-compatible equipment that uses or offers a 3-pin interface. So (and I'm totally speculating here) if DMX doesn't "officially" have a 3-pin option it would seem to me that a box that "officially" supports multiple standards (particularly SACN and ArtNet) is not going to casually blow off the requirements of one of them. But I'm talking out my ass, so someone else might know more about this point.
If you're handy and willing to take on a DIY project, SanDevices (
http://www.sandevices.com/) makes a series of controllers for LED strips that can be reconfigured to output DMX instead of the PCM code that the LED strips use:
http://www.sandevices.com/products.html
I have built both the e6804 and e6802 units (I'm driving 16 universes of LED strips with them in my studio), and I know from personal experience that the 16 outputs of the e6802 can be configured to take ArtNet or SACN and output standard DMX signals. I believe you can do it with the e6804 as well but I haven't personally done that. The e6804s are $99 each, pre built, but if you're pretty handy with a soldering iron you can build your own from their kits. They're fun little builds if you're into that sort of thing. So this is a very cheap option for getting ArtNet or SACN off a cable and outputting DMX. It would require you to do a little cable surgery to connect a standard DMX cable to the unit's Phoenix connectors, but it's fairly easy to do and not a big deal if you're doing an installation and not planning to move the box around a lot. If you are, though, the website has suggestions on outdoor utility boxes (typically used with cable TV installations) that work very well as housings for these units.
Hope that's helpful! Good luck!
Matt