Preparation of sulfonyl chlorydes and mechanism of preparation of tosyl chloride
The way to understand this is to think of the autoprotolysis of water:

$$\ce{H2O + H2O &lt;=&gt; H3O+ + OH-} \tag{1}$$

In general autoprotolysis can be represented in the following form (you can recover eq. (1) by setting $\ce{A} = \ce{OH}$):

$$\ce{HA + HA &lt;=&gt; H2A+ + A-} \tag{2}$$

Now set $\ce{HA}$ as $\ce{ClSO3H}$ (or $\ce{(HO)SO2Cl}$, for clarity) and you have

$$\ce{(HO)SO2Cl + (HO)SO2Cl &lt;=&gt; (H2O+)SO2Cl + ^-OSO2Cl} \tag{3}$$

The cation produced in eq. (3) fragments to give water and the reactive chlorosulfonyl cation:

$$\ce{(H2O+)SO2Cl -&gt; ^+SO2Cl + H2O} \tag{4}$$

The water in eq. (4) reacts with a third molecule of acid in an ordinary acid-base reaction:

$$\ce{H2O + (HO)SO2Cl -&gt; H3O+ + ^-OSO2Cl} \tag{5}$$

The combined equation you have is simply the sum of eqs. (3), (4), and (5).