These reactions are chemical equilibrium. Take, for example,
I
2 ⇌ 2I
This means, that in a given temperature, some I
2 will go to become 2I, and at the same rate, that 2I will become I
2. That was poorly explained, but I hope you understand. I
2 is broken down to 2I at the same rate as 2I forms I
2.
To apply that to your situation, lets add the correct arrows first:
I
2 ⇌ 2I
H
2 + I ⇌ H
2I
H
2I + I ⇌ 2HI
Now, when working with chemical equilibriums, one can add the three reactions to form the net reaction. (We can also multiply the equilibrium constants, and get the K for the net rxn)
Let's imagine we cross out those compounds that are present on both sides of the ⇌:
I
2 is alone on the left side, so we don't scratch that one. The two I atoms can be erased because there's the 2I molecule on the right side. H
2I is also present on both sides, and so we remove them. Now, we're left with the net reaction:
H
2 + I
2 ⇌ 2HI
I hope that helped a little bit. It might not be the best way to try to explain it, but I tried