The type of bark depends upon its genetics. Some varieties have smooth bark, some have bark that peels. If the varieties you planted have the peeling bark, it may not start peeling until the trees are more mature, though.

Anyway, a nicely formed Crape myrtle doesn't need to be pruned at all. At the most, they can benefit from some careful thinning and some removal of lower branches and suckers.

If you posted pictures of your trees, we could offer some more personal advice.

First thing I'd do is select somewhere between 3 and 7 trunks. Having more trunks is too busy looking (at least to me) and tends to distract from each individual trunk (and therefore the bark). Also, by limiting the crape to fewer trunks, each trunk will grow somewhat larger, and do so a little quicker, than for multi-stemmed specimens. Larger trunks will display bark better.

The other thing is to limb up the trunks, eliminating foliage on the lower 1/3 (or at least a few feet) of each trunk. Lower foliage blocks the view of the bark and makes crapes look more shrubbish than multi-trunked treeish.