Lots of differences. 

Both are software defined, but the K3 is of a somewhat more traditional design, employing several roofing filters in lots of configurations. It’s small by some standards, though larger than the KX3. It’s very configurable, and it can have a fully independent second receiver identical in every way to the primary one. Each can have I think up to four roofing filters installed, from as narrow as 200 hz to the 13 KHz one for FM. Of course, DSP is also integral, so it’s not as though you need a 200 hz filter, though it’s helpful I’m sure. 

The KX3, on the other hand, is truly a full software defined radio. The receiver is a direct conversion design, employing only DSP filtering and an optional roofing filter that kicks in for SSB and CW reception. There are no optional filters, since it’s all done in software. It’s very small, roughly 8”X4”X2” and is optimized for easy portable operation. While it only has the one receiver, it can monitor two frequencies at a time, provided they’re within about 10 KHz either side of center. 

Both can be upgraded to 100 watts, although, while the K3’s 100-watt upgrade (and automatic tuner for same) are internal, the KX3’s 100-watt amp and 100-watt antenna tuner are in an external unit that connects to the KX3. 

I’ve never used a K3, but I’d say both are excellent rigs. It really just depends on your needs and maybe on your budget. If you want a portable rig that you can also use at home, the KX3 is a great choice. If you need high performance and need the flexibility of customizing, the K3 might suit you better, though both offer very respectable performance, on par with radios that are much more expensive.
On Jan 18, 2014, at 11:50 AM, richard fiorello <[log in to unmask]> wrote:

> Hi;
> I must confess not an elcraft user but wondering what the 
> differences might be between the k 3  and k x 3? Is one just a 
> newer version?
> thanks
> richard
> sent from my braille note