DSolve[-(n*(1 + n)*y[x]) - 2*(b + (-3 + n + n^2)*x)*Derivative[1][y][x] + (3*a1*a2 + 3*a1*a3 + 3*a2*a3 - 6*(a1 + a2 + a3)*x + 9*x^2)*Derivative[2][y][x] + 2*(-a1 + x)*(-a2 + x)*(-a3 + x)*Derivative[3][y][x] == 0,y[x],x]