Hi
g_ticks(r_1_back_price, r_2_back_price) will give you the difference in ticks between the back price of the fav and second fav.
and selections trigger expression g_ticks(r_1_back_price, r_2_back_price) is greater than <your number of ticks between 1st and 2nd favs>
and selections trigger expression g_ticks(r_2_back_price, r_3_back_price) is greater than <your number of ticks between 2nd and 3rd favs>
or you could simply subtract the prices:
and selections trigger expression r_2_back_price - r_1_back_price is greater than 1.5