These express the number of times something happens, i.e. the number of occurrences. While once and twice are commonly used, and therefore relevant to EFL pedagogy, thrice is seldom used.
They can be used as the denominator to make frequency adverbials, e.g. twice a week or once per day.
Numbers higher than 3 cannot be expressed with single words. Rather, we must use equivalent phrases; numeral adverbial phrases, such as four times, five times, etc. The number 0 is also difficult to express, we tend to use never, though that technically is a frequency adverb, any fraction with 0 as numerator is equal to 0.
Once can mean one time, but can also mean when or as soon as.
In pedagogical grammar these are sometimes misidentified as frequency adverbs, as they have similar grammar. However, in terms of meaning they do not semantically or logically denote frequency, but the number of occurrences of an action. This can perhaps be best illustrated by considering the wh- phrase needed; for number of occurrences it is how many times, but for frequency it is how often.