3 previous c sections pregnant again scan at 6 week showed lower myometrium layer to be 19mm is this to thin?

No, That is not. A typical measurement to take and I am unsure of any data related to outcomes in measuring this, . That being sain it's almost 2 cm which is almost an inch thick. You are likely not going to see thinning in this area until later, as the uterus stretches. I am not familiar with monitoring thickness of this ares, if it gets thin, which it will, what is done differently? Nothing. Do not worry.
See below. Would only be concerned if the placenta at your 30 week scan is lying over this area. Can result in something called placenta accreta. Maybe an OB doc will weigh in on any other issues.