this for to be used on currency exchanges, so im pretty sure it has to be only 3 letters

"Currencies/commodities that don't have a country start with "X" thus

XAU = Gold
XAG = Silver
XPD = Palladium
XDR = International Monetary Fund
XUA = African Development Fund
XBD = European Bonds
XRP = Ripple
XCP = Counter Party
XBT = Bitcoin

And so on. There are actually lots and lots of tickers codes that aren't officially started from a country that start with an "X". "

Is there any source where this standard is defined?

Given 3 letters are allowed, I also vote for XEM.

NEM is an Economy and a Movement. So the E and M are much more representing than the N "new".

For that I would prefer XEM. XEM looks good and sounds good.

logo discussion: http://forum.nemcoin.com/index.php?topic=2228.msg3198#new

+1 on XEM - Love it - Reminds me of another crypto I'm a huge bagholder in, Zimstake - Anyone want to trade some XEM for ZIM? 

-1 on XNM - Burn it - Reminds me of a word jumble and I never find the word

I'm pretty sure you can only cashout your NXT to a credit card, not the other way around.

+4

Separates us even further from NXT.

Unfortunately, four letter codes has never been used. But they may have to open it up one day, when there are too many Xs around. ;-)

I'd say, Play it again, XEM.