Trials are really special. No, really really special. You'll find them in an Austronesian language called Manam, but seldom anywhere else - and never, sadly, in English.
A trial is a sort of word you use when there are three of something.
In English we only get to choose between a singular word (that's if there's just one single thing, for instance dog) or a plural word (for more than one, in which case dog becomes dogs). As you'll have noticed, singulars and plurals usually look different.
If you spoke the Austronesian language called Sursurunga, though, you'd have much more choice. Sursurunga has a singular, a dual* (for two things) a paucal (for a few things) a greater paucal (for a surprisingly large number of things) and a plural, as well.
If you, as an English speaker, are feeling a little limited, take heart. If we were speaking Chinese we wouldn't even have a singular or plural to choose between - though we would have a gorgeously large number of words for some.
Word To Use Today: trial. This word is of course from the Latin trēs and Greek treis, which both mean three.
*Duals are fairly common compared with trials: you'll find them in Scottish Gaelic, Slovenian and Frisian, for instance.