• On this example here is the case:

1. Eth1 connect to ISP1
2. Eth2 connect to ISP2
3. Eth3 connect to Local Network

• First, set the IP addresses:

1. If using DHCP, add them on IP > DHCP Client
2. If set as static, IP > Address List

• Next, add the firewall rule:

1. Click on IP > Firewall > tab NAT

Add (+)
on tab General:
Chain: srcnat
Out Interface: ether1
on tab Action:
Action: masquerade
Click OK

Add (+)
on tab General:
Chain: srcnat
Out Interface: ether2
on tab Action:
Action: masquerade
Click OK

• Set DHCP Server for the Local network.
• Accept the connection, IP > Firewall > tab Mangle:

Add (+)
on tab General:
Chain: prerouting
Dst Address: 192.168.1.0/24 (The IP of ISP1)
on tab Action:
Action: accept
Click OK
Add (+)
on tab General:
Chain: prerouting
Dst Address: 192.168.1.1/24 (The IP of ISP2)
on tab Action:
Action: accept
Click OK
Add (+)
on tab General:
Chain: prerouting
Dst Address: 10.1.1.0/24 (The Local Network)
on tab Action:
Action: accept
Click OK

• Mark the connection, IP > Firewall > tab Mangle:

Add (+)
on tab General:
Chain: prerouting
In Interface: ether1
Connection Mark: 
on tab Action:
Action: accept
Click OK