Code reduction possible with modulo operator?
If your triplets were zero-based (i.e., {0, 1, 2}, {3, 4, 5}, …) then the answer might have been more obvious.  As it is, you have to apply an offset of -1, then divide by three to figure out which triplet an element belongs to.

if ((mvt[i] - 1) / 3 == (mvt[i + 1] - 1) / 3) {
    // This shouldn't happen: mvt[i] and mvt[i + 1] are in the same triplet
}