boiling after 3 miles but...

What are the rads inlet and outlet temps?
If the rad is cooling properly, and in fact it isn't boiling, but is instead building pressure from another source, and when that pressure reaches the cap release pressure, it discharges non-overheated water out the overflow, till, there isn't enuff coolant left, it then starts to overheat ! !
This is the case with most head gasket related overheats..