AC rms value of pulse train vs RMS value
If \$\lambda \in [0,1]\$ is the duty cycle, then the
RMS voltage is \$\sqrt{\lambda} V_\max \$, the DC voltage is \$\lambda V_\max\$ and the
RMS value about the average is \$\sqrt{\lambda(1-\lambda)} V_\max \$.

You have
\$ ( \sqrt{\lambda} V_\max ) ^2 = (\lambda V_\max) ^2 + ( \sqrt{\lambda(1-\lambda)} V_\max )^2\$,
that is, the total RMS squared is the sum of the DC squared and the square of the RMS value about the DC value. The latter works because the DC component and the AC (about the DC value) component are 'independent'.

In the above, \$\lambda = {1 \over 4} \$, \$V_\max=3\$ so we have
an RMS voltage of \${3 \over 2} \$ Volts,
a DV voltage of \${3 \over 4} \$ Volts and
an RMS value about the average of \${3\sqrt{3} \over 4} \$ Volts.