Having 3 treatments represented the typical number of treatments that patients would choose to have in the original Italian pilot studies. A seperate study of patients that had severe vaginal atrophy in breast cancer patients actually followed up for 6 months and 6 treatments - 'standardised questionaires' on bladder and sexual health were completed - and the finding was that there continued to be improvement after the 4th, 5th and 6th treatment anyway.( Now we do not know if the ongoing improvement at months 4,5 and 6 was a delayed effect of the first, second and third treatments or enhanced by the 4th, 5th and 6th treatment)
But to answer your question - no - it is not dangerous.
Whether to have more than 3 treatments though - firstly - had there been improvement with the first 3 treatments - if there was no improvement at all - I would consider an alternative or additional pathology as a cause to your symptoms - ?localised vulvodynia not related to atrophy -- vulval dermatitis for example.
If there had been improvements - including after the 3rd treatment - but you felt there was more improvement to be had then it is reasonable to have more than 3 treatments.