In water, $\mathrm{HCl}$, $\mathrm{HNO}{}_3$ and $\mathrm{HClO}{}_4$ all appear to be equally strong acids. This is called

Water is basic enough to protonate completely any acid stronger than $\mathrm{H}{}_3\mathrm{O}{}^{+}$. All such acids are converted to $\mathrm{H}{}_3\mathrm{O}{}^{+}$, the strongest acid that can exist in water, so they appear equally strong. This is the levelling effect.