The interesting thing about anagrams is how their difficulty increases rapidly with the insertion of even a single new letter. For example, a short anagram like
WROC can be solved within a glance. This is because of the number of ways of ordering the letters is the factorial of the number of letters: there are $4!=4*3*2*1=24$ possibilities to order the four characters before, easily enough to think about and rule out the wrong ones.
However, when given something like
FANGOS THEOREM, there are some 2 billion possible orderings. The length you want should depend on the experience of the solvers with anagrams in the past. I find that most of my audience (a group of a dozen or so friends) can be expected to solve an 8-letter anagram with ease, but I doubt any of them would solve yours without a clue.
Given how most anagrams are traditionally a single word, listing the number of words is an important clue. After that, it would be reasonable to assume the three words form some sort of phrase, either a proverb or media title, so I would pose it as
three word phrase.
It seems you are attempting to use English words when presenting the anagram (judging by your use of
THEOREM). A subtle hint I would use would be to instead provide
AMONGST HEREOF. Note that the second word of the solution is already contained within the clue, even enunciated by the standard pronunciation of
hereof. When considering the anagram, solvers may toy with splitting
hereof into two parts, and the most obvious would give one of the answer words. One issue with this, however, is the more common word
the laying across the gap, so you could re-order the words to
Overall, I'd say
HEREOF AMONGST with the initial clue
three word phrase would be a puzzle that, while difficult, I could expect people to solve before resorting to a computer solver.