All these dictionaries give plurals for English nouns that have irregular plurals. For example, Merriam-Webster here says "plural mediums or me·dia".
If one of these dictionaries does not explicitly give a plural for a noun, the noun has a regular plural, meaning you add an "s" or an "es". The rule is that if a word ends in the sound /s/, /z/, /ʃ/, /ʒ/, /tʃ/ or /dʒ/, you add an "es" unless it already ends with an "e", and otherwise you add an "s".
Notice that this rule depends on the pronunciation. Words like conch that can be pronounced two ways (this is the only one I know of) can have two plurals: conchs or conches.
There are also rules for when you replace "y" at the end of a word by "ies", and when you just an "s", but the cases where you use "-ies" seem to be given explicitly by most dictionaries.