I guess it is a 'changeover' type, this means put the supply to the centre and in one position one outside terminal will be live and when you flick the switch the opposite terminal goes live instead.
According to the latest build manual, you should connect Blue/White to P1 and Brown/Blue to P2. (therefore P3 unused)
Connect it like this before fitting the switch and try it, don't split the blue/whites. I think it will work fine, if it is wrong the high beam will be on all the time and the flasher switch will turn them off! - in this case, move the blue/whites to the opposite terminal.
Hope this helps!
See some photo's of my Roadsport build here