[matteoworld]

ip cef

interface FastEthernet0/0

Description Primary link ISP1

ip address 12.x.x.x 255.255.255.240

ip nat outside

interface FastEthernet1/0

Description Secondary link ISP2

ip address 76.x.x.x. 255.255.255.252

ip nat outside

interface FastEthernet0/1

Description link ISP3

ip address 96.x.x.x. 255.255.255.252

ip nat outside

interface FastEthernet1/1

Description Inside LAN segment

ip address 172.16.20.1 255.255.255.0

ip nat inside

access-list 100 permit ip 172.16.20.0 0.0.0.255 any

route-map isp1 permit 10

match ip address 100

match interface FastEthernet0/0

route-map isp2 permit 10

match ip address 100

match interface FastEthernet1/0

route-map isp3 permit 10

match ip address 100

match interface FastEthernet0/1

ip nat inside source route-map isp1 interface FastEthernet0/0 overload

ip nat inside source route-map isp2 interface FastEthernet1/0 overload

ip nat inside source route-map isp3 interface FastEthernet0/1 overload

ip route 0.0.0.0 0.0.0.0 12.y.y.y track 1

ip route 0.0.0.0 0.0.0.0 76.y.y.y track 2

ip route 0.0.0.0 0.0.0.0 96.y.y.y track 3

Also you could add IP SLA to monitor those links. In case of failure one of the links it will be removed from routing table.

ip sla 1

icmp-echo 4.4.4.4 source-interface FastEthernet0/0

timeout 1000

ip sla schedule 1 life forever start-time now

ip sla 2

icmp-echo 4.4.4.4 source-interface FastEthernet1/0

timeout 1000

ip sla schedule 2 life forever start-time now

ip sla 3

icmp-echo 4.4.4.4 source-interface FastEthernet0/1

timeout 1000

ip sla schedule 3 life forever start-time now

track 1 ip sla 1 reachability

track 2 ip sla 2 reachability

track 3 ip sla 3 reachability