Juan
access-list 101 permit tcp 10.21.0.0 0.0.255.255 host 10.25.5.11
access-list 101 permit tcp 10.21.0.0 0.0.255.255 host 10.25.5.12
access-list 101 permit tcp 10.21.0.0 0.0.255.255 host 10.25.5.13
route-map PBR permit 10
match ip address 101
set ip next-hop 10.25.4.20
int vlan
ip policy route-map PBR
where vlan is the interface that the 10.21.0.0/16 traffic will come in on.
The above will simply send all http traffic from 10.21.0.0/16 to 10.25.5.11/12/13 to the next hop of 10.25.4.20. However this may not be what you want. If you are simply looking to redirect http traffic you may want to look at WCCP.
Jon