"A single atom can undergo only one transition at a time"

I was reading experiments from an old lab manual of a foreign
university that my professor left in my room. One of the experiment is
oberving hydorgen spectrum and studying its details. There is a
footnote in that experiment which says
"N.B: A single atom can undergo only one transition at a time."

Three ignorant questions are:
(i) Where can I find more about this rule?

(ii)This is interesting and we can consider that at a "single instant"
only "one photon" will be absorbed or emitted. My question is that if
we have a SINGLE multielectron atom say Na atom and assume that sodium
atom can absorb light of wavelengths of 300, 400 and 500 nm
corresponding to all allowed transitions. If we were to shine light on
that "SINGLE ATOM" which contains all these three wavelengths
simultaneously, would we observe a SINGLE absorption (due to 300 or 400
or 500 nm) line or three lines at that very instant? If we were to
observe three lines which wavelength would have caused transition
first, since we are assuming that one transition is possible at a time
(or are they equally probable)?

(iii) Another related question is that for a multielectron atom, is it
possible that more than one electron can be excited simultaneously or
there is quantum restriction that only one electron will be excited at
one instant.

Regards.

.

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