During a natural labor and birth following a c-section, the scar where the incision was made on the uterus can rupture during contractions and as the baby moves down the birth canal. This is the main risk associated with VBAC. Of course, elective cesareans also carry their own set of risks, and making a decision is a risk-assessment dance in which it is often not clear which option is safer. One study showed that the risk of uterine rupture increases significantly after two c-sections, though. While the risks of a VBAC vs repeat cesarean section (also look at Should you have a repeat c-section? for more info) may be relatively equal and flipping a coin would not be a terribly irresponsible way to make the decision, this is not at all true for women who already had two, or even more, c-sections.
Few OBs would agree to attend a vaginal birth after two c-sections. That doesn't mean you can't read "VBA2C" stories on the internet, though. Some women do make this choice, and it is possible to have a healthy baby and no ruptured uterus. If you are looking for a VBAC provider after you had two or more c-sections, you are much more likely to find a willing candidate in a homebirth midwife or one at a birth center than a high risk OB. This may be ironic, because a ruptured uterus is no walk in the park and can be lethal for mom and baby. This is probably not something you should be doing outside of a hospital.