It is really all in the semantics. There isn't a difference between the three. Their names are used interchangeably because ultimately they show the same thing. They are called all three ways but they all refer to the same: A list of words that consistently appear in books, textbooks, and stories and that are consistently used in written and spoken language. These are words whose constant exposure make it easier for children to read once they are used to recognizing them. The reason why they call them Dolch is because Dr. Edward William Dolch put a first list of these constant words together back in the 1930's. Yet, they are no different than the words that are most consistently showing in print.
It is actually a common misconception that sight words and high frequency words are the same thing. This is easily understood as we instruct them in a similar fashion when working with beginning readers, and several words can be found on both sight word lists and high frequency word lists. However, there are subtle differeneces between the two groups of words that can provide distinction.
Sight words are defined by recognition. Sight words are words that the reader can recognize "at first sight," therefore readers do not need to devote cognitive energy to decoding or encoding these words. When working with students in an intervention setting, I often refer to these words as "words we know by heart" or "our word bank."
High Frequency words are definied by usage in texts. High Frequency words are the words that appear the most often in written text in the English language. We want students to memorize these words, as they are encountered over and over and over while reading. While many of these words do not fit our standard phonetic patterns, some do. The word "the" would also appear on this list, as it is used extremely often in the written word. (I've used it twice in this sentence.) This crossover between high frequency words and sight words is why many people them to be the same thing.
Finally, Dolch words are defined by Dr. Edward William Dolch as the words needed to instruct children through the "whole word movement" which has now been discredited and replaced with phonemic awareness and phonological awareness instruction (See the National Reading Panel Report of 2000). When compiled, they were thought to be the words most frequently used in written texts (the high frequency words of their day), and the words that Dolch thought would be most appropriate to memorize as whole words in order to learn to read. However, this was in 1936, and research now supports instruction in phonics to teach children to blend and segment sounds within words.
Sight words are those recognized immediately by the reader without having to break them down into constituent parts. Many of these words do not follow basic phonological rules and are learned through rote memorization and practice. High frequency words are the same thing and as noted above Dolch created one of several lists of the most commonly used words in printed English. The Dolch list consists of 220 words which cannot be learned though pictures. These words make up 50-75% of printed material. The link below provides a list.
Essentially all three terms mean the same thing. Doesn't matter what you call them they are the words that appear most in print and are taught through rote memorization. I've taught kindergarten for many years and sight words are reviewed on a daily basis through many means and are taught using a variety of materials. I have created sight word movies, powerpoints, flashcards, word walls, smartboard activities, etc. to promote the learning of these words in my classroom. The more sight words you know the faster your reading fluency will be.