The three fold law is a belief not a fact there are many potential "laws" that are dependant on someone's religion, eg: karma and the three fold law.
It is very very unlikely that you would have boils after doing a spell your symptoms will be caused by something else and should be seen by a doctor to find the cause.
Depending on your knowledge level the lack of results may be due to a lack of knowledge and skill on your part. The spell may also not have worked because it is not meant to work, spells can help assist in healing but they will not fix it instantly. If they are are very ill and are being treated by professionals then the most you could do is a spell to assist in the speed on healing.
Spells more frequently backlash in individuals who are less skilled or starting out. As you learn more you will find ways to avoid any negative implications from spells and rituals in the future.