It is true. The use of the words "three suns" to mean "three days" is an example of metonymy.
Metonymy is a figure of speech in which a thing or concept is replaced with a word closely related to that thing or concept. Metonymy only works when there is contiguity between two concepts. In this case, 'sun' is closely associated with our concept of 'day' since the sun produces a daily solar arc and moves across the sky each day. Furthermore, the earth rotates about its axis, which leads to the day and night phenomenon. Therefore, the concepts 'sun' and 'day' are closely related, and, for that reason, 'sun' can be used to stand for 'day'.
