3. Non-zero integers $a, b, c$ are three consecutive terms of a decreasing arithmetic progression. All six quadratic equations, whose coefficients are the numbers $2a, 2b, c$, taken in any order, have two roots. Find the greatest possible value of the common difference of the progression and the corresponding numbers $a, b, c$.
Answer: $d_{\max }=-5 ; a=4, b=-1, c=-6$.