There are agent nouns for all genders.
For example, saltare gives rise to saltator, saltatrix, and saltatrum.
For more details, see this question.
The stem is revealed by the genitive form.
For my three examples they are saltator- (third conjugation), saltatric- (third), and saltatr- (second).
(The stem of rex is reg-, so it has a g instead of a c.)
If you want to use the usual -ulus/-ula/-ulum for diminutives, it would have to be attached to these stems.
So the natural guesses would be saltatorulus, saltatricula, and saltatrulum.
Then it remains to check whether such diminutives are actually in use.
The word saltatricula is post-classical but attested.
The given translation is, unsurprisingly, "little dancing girl".
For another similar example, cicatricula is a diminutive of cicatrix, and also nutricula comes from nutrix.
These make a decent case for diminutives of words ending in -trix, including feminine agents.
I found no examples of -torulus (apart from the word torulus, a small torus).
A different kind of diminutive suffix might work, and -torculus sounds more natural to me.
Indeed, it does exist: amatorculus.
I found no other examples though, so perhaps there is another way or this just is so rare.
If you need a diminutive of a masculine agent, I suggest -torculus.
I found no examples of -trulum or -trellum.
They both sound reasonable to me so I would go with either of them, but I have not managed to find any examples of such words in a dictionary.
So, the three little dancers should probably be saltatorculus, saltatricula, and saltatrulum or saltatrellum.
Such diminutives are rarely pre-existing words, but diminutives and agents are productive in Latin and especially in proper context such words are easily understandable.
