Adding 2 or 3 feet of soil over the roots is almost certain to kill those roots. If you kill a significant part of the tree's roots, the tree will suffer and likely die prematurely.

I agree with Brandon.

You COULD safely add up to 4 inches of soil over the roots, but be aware that over time, the tree's feeder roots will grow up into the new soil, especially if you are watering there.

I would suggest instead that you plant something that is tough enough to compete with the roots, will spread (which the liriope will do only very slowly) to hide the visible surface roots, and will do OK on its own in dry shade, after initial watering for establishment, such as pachysandra or an attractive ivy - much though I dislike ivy in most situations. If you don't let the ivy grow up the trunk, and keep the edges trimmed back, you can contain it to the area just around the tree. Using the native Allegeheny pachysandra instead of the Japanese one might be nice, if you can find it.

Mulching around the plants when you put them in will also help, if you lay down a fairly fine mulch so any new roots can take hold in it (ivy and pachysandra both will grow new roots where the stems touch the ground) - it will help keep the area more damp. The usual mulch mantras apply - as widely as possible, no deeper than 4", and no mulch in the 2-3" next to the trunk.

I wouldn't do it. I'd suggest lightly mulching around the tree and putting the liriope on the edge of the mulch. You don't even have to disturb the soil all that much. Liriope grows like a weed down here and in a few years a ring of small sprigs will create a natural border around your tree. I did it with a dogwood in our yard and it looks nice....especially come august. No rocks needed.