Had 3 CT scans over the last 15 mons after an infection from a dental procedure. Today I was told I will most likely get brain cancer- is it true?

No. risk of brain cancer after exposure to this amount of radiation is relatively low. It is far more important to be sure the infection has been treated successfully.
Run. Run, as far away as you can, from whoever gave you the misinformation that properly administered medical tests cause brain cancer. Also, I doubt that your dental procedure caused infection. More probably you had a dental infection that was beyond the scope of the procedure to cure. You had appropriate testing to asses both the extent of the infection and the treatment effectiveness.