Traveling north on i-15 between mile 120 and 121 near manderfield utah. looking nw, near granite peak, three bright lights in a tight triangle shape clearly observed. lights hovered for approximately 3-4 seconds while i was looking at it - at this time, two other aircraft at a higher elevation and quite a distance away were also observed. (not a commercial aircraft area; but these two were clearly planes). the three tight triangle lights then appeared to fall rapidly to a diagonal, still remaining in the nw sky, but quickly losing elevation. after approximately 4 seconds of falling, the lights all completely disappeared.