In the fundamental mode or case 1 as shown above There are one node at the center and two antinodes at the open ends.
We can see that $$\dfrac{\lambda_1}{2} = L$$, where $$L$$ is the length of pipe.
Let the velocity of the sound wave be '$$v$$'
Then frequency $$\nu_1 = \dfrac{v}{\lambda_1} = \dfrac{v}{2L}$$
For second case similarly we see
$$\lambda_2 = L$$
So, $$\nu_2 = \dfrac{v}{\lambda_2} = \dfrac{v}{L} = \dfrac{2v}{2L}$$
For third case
$$\dfrac{3\lambda_3}{2}=L$$
So, $$\nu_3 = \dfrac{v}{\lambda_3} = \dfrac{3v}{2L}$$
Similarly if we take for $$n^{th}$$ harmonic we will get
$$\nu_n = \dfrac{v}{\lambda_n} = \dfrac{nv}{2L}$$
We can see from above that all the harmonics
$$\dfrac{v}{2L} , \dfrac{2v}{2L}, \dfrac{3v}{2L}.......\dfrac{nv}{2L}$$
is present in case of air column vibrating in a pipe open at both ends and it can be explained because '$$n$$' can take any positive integer value from $$1$$ to infinity.