multi internet line about redundancy

1.i have three internet line from isp,i config it as a pool and give the three members for differents priority . 2.default monitor methods:gataway_icmp 3.sometimes gataway is up ,but i can not access internet through the line ,so i wanna changeto the default monitor methods from gataway_icmp to dns montor(if dns can reach,internet is ok) 4.the three isp have three differents dns question: what can i config it ,please help me,thanks!