boiling after 3 miles but...

When OP says boil, I read pukes to atmosphere, so 3 miles sounds about right to me at this time of year, even in Sweden,for the stat to open. The coolant in the rad will still be cold. When the hot coolant finally bursts thru the stat at hiway speed, the coolant will all mix pretty much instantly and expand , blowing the excess into the overflow tank.
If it pukes to atmosphere, you have the wrong cap, the wrong rad, or a defective cap/rad sealing surface.
But yes, the stat could still be lazy.