When adding a verb in the infinitive after another verb, here gouter after il faut, I have noticed that sometimes there is a ‘en’ or a ‘de’. Is there a rule for this? If it were left out would the sentence still make sense?
Some verbs and phrases require a preposition (à or de) after them before an infinitive. There are lots of sites, videos, etc. about this. For example: Want to Say 'of' in French? Use the Preposition 'de'.
But that is not what “en” is here. It is replacing a non-existing noun in that sentence (something we don’t have to do in English).
il faut en goûter absolument au moins trois = you absolutely have to taste at least three [of them].
In English we don’t have to say “of it/them” but in French you do and that’s what the “en” is for. Lots of helpful info online about all the uses of “en” (and “y”) that can probably explain it better than me, such as: https://www.talkinfrench.com/french-pronoun-en-y-use-grammar/.