What basic addition is the Pasuk coming to make to Pasuk 1 - 3?
Seforno: Until now, the Pasuk was referring to destruction of Par'oh and his horse exclusively; whereas now, it deals with the destruction of Par'oh's army and elite officers (together with their chariots). 1
The previous two Pesukim comprise thanks for the former aspect of the miracle, Pasuk 6, for the latter.
Why does the Torah refer to officers as 'Shalishim'?
Because the number three (Sheloshah) denotes strength. 1
I once read that the triangle is the most popular shape used in building bridges, because it is virtually impossible to knock it out of shape. It is also noteworthy that Yisrael descends from the three Avos. (EC)
What are the connotations of "Tub'u be'Yam Suf"?