3x4 is an example of a character pairing from the series Gundam Wing: an anime. The '3' represents the pilot Trowa Barton and the '4' represents his fellow pilot Quatre Raberba Winner.
These two pilots are male thus this pairing is also an example of a yaoi(i.e., shounen-ai, boys love & the Western genre of slash) pairing. Their relationship--though some will say otherwise--is not fully canon and is mostly built up by fanfiction authors and readers, as they enjoy viewing the two as a couple.
Due to the '3' being in the first position that makes Trowa the semeand or top(the one who penetrates the other male) in the relationship. Quatre written as the ukeor bottom. Were this pairing flipped, that is: Quatre being the seme, it would be written as 4x3 rather then 3x4.
A (relatively) full list of the number and character representations from Gundam Wing are as follows:
01 = Heero Yuy
02 = Duo Maxwell
03 = Trowa Barton
04 = Quatre Raberba Winner
05 = Chang Wufei
06 = Zechs Merquis (i.e., Milliardo Peacecraft)
09 = Lucrezia Noin
13 = Treize Khushrenada
Those numbers can be used to create any number of pairings in the Gundam Wingfandom. The most popular of the yaoipairings being 1x2 followed closely by 3x4 and then other miscellaneous pairings such as 5x2, 13x6, 3x2 and 2x4.
There are more number representations, but the characters are minor and are more often then not left out of most fanfiction. When placed in fanfiction because of their rarity authors will often just write out the character's full name so as not to leave people confused.
"My favorite pairing is 3x4 because I'm obsessed with how fluffy their relationship can be."