Dynamic Active/Active/Standby Load Balancing

Load balancing from a single virtual server to multiple pools can be accomplished in a bunch of ways, and more or less depends on how you determine the load balancing selection. For example:

when HTTP_REQUEST {
     switch -glob [string tolower [HTTP::uri]] {
          "/foo*" { pool foo_pool }
          "/bar*" { pool bar_pool }
          default { default_pool }
     }
}

This example would assume that the client was requesting a specific URI (the trigger), so you're implementation may be a little different. In any case, because you've defined different pools with their own priority groups, something like this should meet your requirements. You can also, as you mentioned, load balance from one VIP to three other VIPs, and is an excellent solution if you have to apply different profiles and/or iRules to the traffic.