Learn English – Singular vs plural noun following list

As the grammatical-number tag you have added implies, this is a matter of the 'number' of the word. As the dictionary definition says, trio is a singular noun, even though a trio is made up of three objects or people. So it is correct (and sounds correct to me) to use "was" with it instead of "were".

In general, collective nouns such as "trio" take singular forms of the verb, and this is the "correct" - or at least traditional - usage. For example, "The flock [singular] of sheep is in a field" vs. "The sheep [plural] are in a field".

However, in recent years this has begun to change in some circumstances. The examples which I most commonly hear are ones such as, "The government have made a new policy" and "England have beaten Germany in the football championship". (The last of these being both grammatically peculiar, as well as unlikely.) Both of these sound wrong to me but are increasingly used, including by the BBC, so should probably now be considered part of standard English. (See: https://en.wikipedia.org/wiki/Collective_noun#Metonymic_merging_of_grammatical_number).