Hello,
here is a sample configuration using static routes. The first static route is for the primary link, the secondary static for the secondary, the third static route for the tertiary link. The logic is: if link 1 fails, link 2 takes over. If both links fail, link 3 takes over.
IP addresses are obviously arbitrary, yours will be different. IP SLA 1 tracks your primary link, IP SLA 2 tracks the secondary link.
track 1 ip sla 1 reachability
!
track 2 ip sla 2 reachability
!
track 3 list boolean or
object 1
object 2
!
ip route 0.0.0.0 0.0.0.0 192.168.12.2 track 1
ip route 0.0.0.0 0.0.0.0 192.168.13.3 track 2
ip route 0.0.0.0 0.0.0.0 192.168.14.4 250
!
ip sla 1
icmp-echo 192.168.12.2 source-interface GigabitEthernet0/0
frequency 5
ip sla schedule 1 life forever start-time now
ip sla 2
icmp-echo 192.168.13.3 source-interface GigabitEthernet0/1
frequency 5
ip sla schedule 2 life forever start-time now