hello,
i have router board 450
i have 3 lines on 3 routers
what is the best way to load balance them on RB450
and put pppoe-server on the Lan interface .
thanks
You should start here.
I followed this guide and it worked great for me 
http://aacable.wordpress.com/2011/08/29/mikrotik-4-wan-load-balancing-using-pcc-with-pppoe-server-complete-script/