You have two possible ways to connect the springs: in parallel or in series.
The equivalent stiffness of three springs in parallel is given by:
k_eq = k₁ + k₂ + k₃
In order to keep this number the smallest possible, you need to take the three springs with smaller k:
k_eq_min = 3.5 + 6 + 8.5 = 18 N/m
The equivalent stiffness of three springs in series is given by:
1 / k_eq = 1 / k₁ + 1 / k₂ + 1 / k₃
In order to get the smallest k_eq possible, 1 / k_eq must be the biggest possible, therefore you need to take again the three springs with smaller k:
k_eq = 1 / (1 / k₁ + 1 / k₂ + 1 / k₃)
= 1 / (1 / 3.5 + 1 / 6 + 1 / 8.5)
= 1.754 N/m
Therefore, in order to get the smallest equivalent stiffness, you need to connect the first three springs in series (one after the other).
