It's the other way round. 'Viande' and 'fromage' are used as uncountable nouns here and are therefore in the singular form, while 'légumes' are countable, and therefore in the plural form with an 's'.
"On mange de la viande (uncountable, singular, feminine) du fromage ( uncountable, singular, masculine) et des légumes. (plural)"
It's the same as in English - some meat, some cheese, some vegetables.