WHY ARE THERE 3 TYPES OF ADVERB OF FREQUENCY ?
There are only two types of adverbs of frequency
(a) definite adverbs of frequency: weekly, monthly,daily, etc.
(b) indefinite adverbs of frequency: often, sometimes, usually, occasionally, etc.
There is no specific reason as to why there are two such types. Adverbs of frequency answers ' how many times' the action is performed. Most of the adverbs of frequency fall under these two broad categories. Hence, there exist only two such categories.