The Radial J48 (active), JDI(passive)  can merge a stereo signal into an xlr mono. Buy two if you want real stereo (that is how most of the live soundguys do it).

But if you want stereo with your x3 then I'd suggest the Radial Pro D2.

This one looks cool as well: http://www.radialeng.com/re-duplex.htm. Seems very versatile.

All of these DI's have a somewhat high price tag, but they are recognized in the bussiness... In general you want to have the x3 electrically isolated from the mixer to avoid any ground loops that WILL occur when using non-battery powered stuff from an outlet different from the mixer.

In general active DI's are for passive devices that doesn' put out too much output like passive basses and such, otherwise they may distort with too high input. They will also need +48v of phantom power (som may be able to use batteries as well).

Passive DI's can be used by almost anything and doesn't require phantom power but have other drawbacks that the active doesn't have.

A DI can make or break your tone so make sure you test it (if it's very cheap and of dubious quality) and verify the low-end. This is what mostly seperates the good from the bad ones.