To the best of my knowledge, there is no K-3xe, (only a K-3x version), as Ayre discontinued the K-3 line prior to the "e" (evolution) upgrade becoming available. The reason the K-3 line was phased out was that it was pretty expensive to produce, given its similarity in construction to the K-1 line.
From what I can read and interpret from Charles Hansen's posts on Audio Asylum, the K-5xe is very close to the K-3x. (Maybe even slightly better, but very close sonically.) FYI, there is a new version of the K-5 called the K-5xe mp, which supposedly is even better, and probably clearly better, sonically speaking, than the K-3x. (Note however that the K-5 line of preamps is a line stage only, whereas the K-3 does have a (supposedly) very good, albeit optional, phono stage.)
Used, the K-5 preamps typically run around $2K or so. If you are willing to stretch your budget up to around $4K, you can get a used K-1xe (line stage only) for that money. (A K-1xe with the phono stage will run a bit more, to perhaps $4,500 or so.) FYI, there is no mp upgrade for the K-1xe, due to its entirely different design from the K-5 line.
FYI, I own the K-1xe, with the phono stage, and it is a reference preamp with a near reference phono stage. I have been very happy with it for about five years now.
If you can get a used K-3x, with the phono stage, for around $2.5K or so, I think you should take it. (However, if you only run digital, and don't need the phono stage, you're probably better off with a used K-5xe.)
My two cents worth anyway.
By the way, here is a link to some of Charles Hansen's thoughts on the matter:
Charles Hansen's post on Audio Asylum
From what I can read and interpret from Charles Hansen's posts on Audio Asylum, the K-5xe is very close to the K-3x. (Maybe even slightly better, but very close sonically.) FYI, there is a new version of the K-5 called the K-5xe mp, which supposedly is even better, and probably clearly better, sonically speaking, than the K-3x. (Note however that the K-5 line of preamps is a line stage only, whereas the K-3 does have a (supposedly) very good, albeit optional, phono stage.)
Used, the K-5 preamps typically run around $2K or so. If you are willing to stretch your budget up to around $4K, you can get a used K-1xe (line stage only) for that money. (A K-1xe with the phono stage will run a bit more, to perhaps $4,500 or so.) FYI, there is no mp upgrade for the K-1xe, due to its entirely different design from the K-5 line.
FYI, I own the K-1xe, with the phono stage, and it is a reference preamp with a near reference phono stage. I have been very happy with it for about five years now.
If you can get a used K-3x, with the phono stage, for around $2.5K or so, I think you should take it. (However, if you only run digital, and don't need the phono stage, you're probably better off with a used K-5xe.)
My two cents worth anyway.
By the way, here is a link to some of Charles Hansen's thoughts on the matter:
Charles Hansen's post on Audio Asylum