The 3rd stride of viShNu as well as his snake is the enduring Hindus symbol of the axis that precesses with the passage of kAla or time. Right from the R^igveda this axial role of viShNu is at the fore in all conceptions of him. We have alluded to this a few times before, see: snake of viShNu, the Finnish epic, and more obliquely in fall of praHlAda. The element of the axis and its precession is very much a part of the “core mythology” of viShNu: 1) the horn of matsya- the new axis being provided for the world after its shift due to precession. 2) The mandAra mountain as the staff for the churning of the world ocean being borne by viShNu on his back in the form of kUrma. 3) varAha raising the “world plane” with his tusks- the world plane being the equatorial circle intersecting the ecliptic. It had sunk because the precession had pushed the old intersection with the ecliptic below the plane. 4) nR^isimha emerging by shattering the pillar- the old axis is broken by the flow of time by precession. 5) trivikrama with his three steps stabilizes the new world plane (equatorial circle), the ecliptic’s high point (step 2) and the precessing axis (step 3). As the knowledge of precession is very subtle (even alluded in the opening of the maitrayaNi brAhmaNa fragment by shAkAyanya who was well aware of these matters) the third step of viShNu is secret and was only know to the great kavi-s.
In Indian architecture its depicted more directly on many occasions, but more subtly in the great iron pillar of the guptas. Before the iron pillar was uprooted by the Mohammedans and placed in the vicinity of their giant phallic signpost at Delhi, it graced the now derilict Hindu astronomical observatory at the viShNupAda-giri in the udayagiri complex. The iron pillar (as shown by Balasubramanian) was graced by a nakShatra-chakra on the top and was known as the viShNu dhvaja– depicting the great axis in front of the observatory located atop the hill. Thus, in the immortal pillar did chandragupta II vikramAditya, one of the greatest rulers of bhArata commemorate the step of viShNu and and world axis.
This was also very much in the mind of daNDin when he composed his pUrva pIThikA:
brahmANDa Chatra daNDaH
shata-dhR^iti bhavanAmbho-ruho nAladaNDaH
tribhuvana vijayastambho .a~Nghri daNDaH
shreyas-traivikramas te vitaratu vibudha dveSiNAM kAla daNDaH ||
The rod of the umbrella cap of the universe,
the reed stock for the lotus seat of the one of 100 sacrifices,
the mast rod of the ship that is the earth
the rod for banner, that is the heavenly stream (the Milky Way),
the axis of the rotating star-wheel,
the rod that is the victory pillar of the 3 worlds, may the foot [of viShNu] favor you,
the rod that is the leg of trivikrama with which he strode to conquer the 3 worlds,
is the rod of time (death) to the enemies of the devas.
Recall the R^igveda: atha abravId vR^itraM indro haniShyan | sakhe viShNo vitaraM vi kramasva ||
Indra spoke as he set out to slay vitra: “friend viShNu stride widely”. In the Germanic world likewise we have the cognate of viShNu, viDAr (=vitaraM; compare with viShNu=wide) taking his 3 steps before killing the Fenris wolf and the time of the world-end chaos or the rAgnarok. daNDin of Kanchipuram was remembering a very ancient motif and artfully weaving his own name into the pUrvapIThikA of dasha-kumAra-charitra, a literary work beyond compare. The whole work does not survive in its original. While I am told a medieval Telugu translation was made I am not much aware of it to say if it preserve more of the original.