Anyway to config a "backup" parent?

Hi Squider,

    Let me describe my configuration first.

    P1 P2 /
     | | P3 --
      \ / /
       A ---'

    My Squid server "A" has 3 parents, P1, P2 & P3
    P1 & P2 are nlanr's servers, P3 is another server in local.
    I had enable the ICMP and PING futures..
    Now, my problem is, how can I make P3 just be a "backup" parent?
    Because I found that, FIRST_PARENT_MISS will go to P3, but this
    is not what I want, P3 has a slower link connected to outside.
    The "weight=x" option don't work if the site is not in netdb yet.
    It seems that, Squid just pick a parent in random if that site
    is first time accessed and no UDP_HIT from sibling...

    And, 2nd question.
    After carefully read the "Network Probe Database" from cachemgr,
    I am confused by the "RTT" and "Hops" values..
    I want to know, how does my Squid use the value to do the weight
    counting?
    (1) The value from my Squid to them (P1,P2,P3) plus from them
        to the destination?
    (2) Just what the value I saw in cachemgr..
    Or, the value I saw in cachemgr is equal to (1)???

    Thankx for anybody who can tell the answer...

--
Alex Lau