The clearest examples of third-declension adjectives that decline as consonant stems in the neuter nominative/accusative plural seem to be the following:
the class of comparative-declined adjectives, such as neuter plural nominative/accusative adjectives ending in -iōra (including maiōra/majōra) and minōra
plūra (despite genitive plurium). It seems pluria was maybe a very uncommon variant form, but the typical form was plura. The derived word complura/compluria apparently exists in both forms.
ūbera. Of course, this exists as a plural substantive, but according to Allen and Greenough §119, it also exists as a neuter plural adjective (although the ablative singular form of this adjective is usually ūberī). Wiktionary gives "ūberia", but that form doesn't show up anywhere in the PHI Latin Texts corpus, so I'm guessing Wiktionary is just showing an incorrect declension table for this word.
Less clear examples
I have found an old book ("Some improvements to the art of teaching...," Fifth Edition, by William Walker, 1693) that also mentions compound adjectives ending in -corpora or -colōra, like bicolōra, but it seems to say that these actually originated as o-stem forms (corresponding to masc. singular nominative -corporus, -colōrus), not as consonant-stem forms.
I found a few more mentioned in §3 of "Declension of the Latin present participle in connection with its syntactico-semantic use" (2018), by Hendrik Christiaan Walvoort.
Walvoort says that in a discussion of words that can take -i in the ablative singular but that have plurals forms in -a, Priscian mentions ludicra, which is supposed to go with the ablative form ludicri. However, it seems that Priscian is the main evidence for this as a third-declension adjective; dictionary entries record instead a first/second declension adjective ludicer or ludicrus.
Walvoort also provides a citation indicating that present participles had neuter plural forms in -a instead of in -ia in rare contexts:
Kühner and Holzweissig [...] consider the ending -a as a striking singularity instead of -ia in such phrases as silenta loca and pestilenta loca.