Fox's commentary tells us that the poem, in which three talking skulls warn us of our impending doom, is likely influenced by the legend of the Three Living and the Three Dead (Fox, Denton, ed. The Poems of Robert Henryson. Oxford: The Clarendon Press, 1981. p.487). Suffice to say here that the chorus of three dead voices (not literally heard in this poem , but well represented) proves chilling to the extreme. It is perhaps the feeling that the solitary reader, the sinfull man, is outnumbered that gives their number some extra power.
The skulls warn us to flee fra wicket vycis, and in the poem the florid language of growth and life is juxtaposed with grizzly descriptions of the decayed empty skulls. One gets the feeling that despite the moral tone, Henryson must have rather enjoyed the alliterative death-descriptions:
Full laithly thus sall ly thy lusty heid,
Holkit and how, and wallowit as the weid...
... As we ly thus, so sall ye ly ilk ane,
With peilit pollis, and holkit thus your heid. (20-21, 31-32)
The poem neatly reminds us of the folly of human pride; beautiful ladeis and perfyt palmester alike will some day join this chorus. Henryson's message is not stunningly original and he does not labour the point. The poem is to the point, and unsettling.
We are left with a rather shocking image however. It is one which leads one to ask whether Henryson really intended the inevitable comparison, as the three skulls pray in the name of the holy trinity:
With the hie Fader be eternitie,
The Sone alswa, the Haly Gaist conding,
Thre knit in ane be perfyt vnitie. (62-64)
The image of three skulls chanting seems familiar to me but I'm not sure why. I'm sure I've seen an old cartoon with just such an image (the film version of Orwell's Animal Farm has an image of pigs which seems quite similar). A Youtube hunt lies ahead.