The most important measurement is the bolt circle diameter or BCD. This is the spacing that the mounting bolts use. On a 3x crank there should be 2 BCDs, one for the inner 'granny' ring mounts and one for outer mount used by the middle and outer ring. There are a variety of BCDs is use (see http://www.sheldonbrown.com/cribsheet-bcd.html). The standard is 104 for the middle/outer rings and 64 for the inner ring.
You should also make sure that the number of bolts matches, older cranks used 5 bolts were most modern cranks use 4 bolt.
You don't say how old your crankset is so I'm going to base recommendations on the current product listed on SRAM's site (https://www.sram.com/sram/mountain/products/x7-crankset). The use the 104mm / 64mm BCD measurements.
If you are happy with your current ratios then I would suggest sticking to those chainring sizes. The SRAM site lists this cranks as having 44t/33t/22t, however I'd not be surprised to find a 32t in the middle as this is a more common size.
You don't mention which rings are worn. Most of the wear tends to occur in the middle ring as this is what you spend most time using. If only one ring is worn then just replace the worn one.
Material wise you have the choice of steel or aluminium. Steel is heavier but will last longer, while alloy is lighter. Match the new ring to what you have now ideally (use a magnet to identify steel if you are unsure from appearance).
You should be able to get exact replacement parts from your local SRAM dealer, the sram site above links to a spares catalogue which gives a part number. However if you are looking for upgrades or to pimp your bike a little then Raceface, Middleburn, Hope and Blackspire make nice products (at a price though compared to a direct replacement).
If you are keeping the triple set-up then you can ignore narrow-wide rings as these are target at single ring set-ups. As you have a front derailleur there is no benefit to you in choosing a narrow-wide ring. The same can be said for elliptical rings, they won't play nice with your inner and outer ring.