Jatin,
The conventional approach to testing linear trend with 3 levels, assuming equally spaced, is to assign weights of -1, 0, and 1. If you are concerned that the middle level is not really considered in the test, you can run two extra tests, one with High vs. Medium, and the other with Medium vs. Low. The combination of the 3 tests should be good enough to identify linearity. In the meantime, add a quadratic test with weights of 1, -2, and 1, and see if quadratic relation exists among the 3 levels.
Gang
