boiling after 3 miles but...

Just overfilled is all? Or defective radcap?
With a recirculating type system
When the coolant gets hot it expands,and about a half a liter is forced out of the rad into the expansion-tank;this is normal. Then when the engine cools, the contracting fluid pulls the previously pumped out fluid back in. The rad cap is in charge of this event,
and the hose between the filler neck and the expansion tank, and into the tank to the bottom of said tank, has to be one continuous system in order for this to work. If the contracting fluid pulls air in instead, eventually you will have an air pocket in the top of the rad about 1 inch deep. If your rad is a 1970 or older balloon-top design, it was meant to operate like that so it's no big deal. In about 1972 or 73 the rads were redesigned and had smaller tanks. These rads will still operate this way, with the air-pocket at the top, with a stock engine. As the engine performance increases, the engines tend to run warmer, and if you don't keep an eye on the fluid level, you could (not will) eventually get into an overheat situation. At 420 hp, I haven't yet had a problem with my 1973 square top rad, operating in the old mode where the expansion tank is just a puke-tank. And I run a 7psi non-recirculating type cap. The top tank in my system has become the recirculation tank.