If you want to match on the host-header, following config should work:
serverfarm host SERVERFARM1
...
class-map match-all VIP-SITE
2 match virtual-address 192.168.160.1 tcp eq www
class-map type http loadbalance match-any SITE1
match http header Host header-value http://www.site1.com
class-map type http loadbalance match-any SITE2
match http header Host header-value http://www.site2.com
class-map type http loadbalance match-any SITE3
match http header Host header-value http://www.site3.com
policy-map type loadbalance http first-match SITE_L7_POLICY
class SITE1
serverfarm SERVERFARM1
class SITE2
serverfarm SERVERFARM1
class SITE3
serverfarm SERVERFARM1
policy-map multi-match VIPs
class VIP-SITE
loadbalance vip inservice
loadbalance policy SITE_L7_POLICY
loadbalance vip icmp-reply active
HTH,
Dario