According to the regular stress rules from Appendix E of The Lord of the
Rings, all three of these words should be stressed on the penultimate
(second) syllable, since that syllable is followed by two consonants;
however, the way they are used in poems suggests they should actually be
stressed on the antepenultimate (first) syllable (example: “in IMlaDRIS
it DWELLS”). According to Concerning Stress Placement In Sindarin [1],
this can be attributed to a hypothetical “muta cum liquida” rule like in
Latin, where some syllables count as light even if they’re followed by
two consonants, because both consonants belong to the following syllable
(e.g. IM-la-dris instead of im-LAD-ris). However, the rule is apparently
not applied systematically: according to [2], in another poem the word
“nogothrim”, which is quite similar in structure to “Nargothrond”, is
clearly stressed on the penultimate syllable. For this reason, we don’t
try to define the “muta cum liquida” rule in sjn_rules (or qya_rules),
but only add the three names Imladris, Menegroth and Nargothrond as
exceptions to the sjn_list.

[1]: https://menegroth.github.io/stress-in-sindarin.html
[2]: https://mildred-of-midgard.dreamwidth.org/158375.html#comment-cmt49575