The given setup is in equilibrium springs were in natural length before force was applied. If I have to replace the above setup of 1 block and 2 springs by 1 block and 1 spring, What will be the spring constant of that new spring, such that the block is displaced by the same amount as previously.
K3(K1+K2)K1+K2+K3
Spring1 and spring 2 will have same expansion so they are in parallel.
2 parallel spring with spring constant K1 & K2 can be replaced with 1 spring of constant (K1+K2).
Now this new spring will be in series with the spring 3.
So for series equivalent is
1K1+K2+1K3=1Knet
Knet=K3(K1+K2)K1+K2+K3
Alternate solution
X3=FK2 .....(i) K3X3=(K1+K2)X1=F ⇒X1=FK1+K2 ..........(ii)
F=K0Xnet
X0=FKonet .......(iii)
X0=X1+X3
⇒FK0net=FK1+K2+FK2
1Knet=1K1+K2+1K3
Knet=K3(K1+K2)K1+K2+K3