/ip firewall mangle
add action=accept chain=prerouting dst-address=192.168.1.0/24
add action=accept chain=prerouting dst-address=192.168.2.0/24
add action=accept chain=prerouting dst-address=192.168.3.0/24

add action=mark-connection chain=prerouting connection-mark=no-mark in-interface=ether5-LAN new-connection-mark=ISP1_conn passthrough=yes per-connection-classifier=both-addresses-and-ports:3/0
add action=mark-connection chain=prerouting connection-mark=no-mark in-interface=ether5-LAN new-connection-mark=ISP2_conn passthrough=yes per-connection-classifier=both-addresses-and-ports:3/1
add action=mark-connection chain=prerouting connection-mark=no-mark in-interface=ether5-LAN new-connection-mark=ISP3_conn passthrough=yes per-connection-classifier=both-addresses-and-ports:3/2

add action=mark-routing chain=prerouting connection-mark=ISP1_conn in-interface=ether5-LAN new-routing-mark=ISP1_table passthrough=yes
add action=mark-routing chain=prerouting connection-mark=ISP2_conn in-interface=ether5-LAN new-routing-mark=ISP2_table passthrough=yes
add action=mark-routing chain=prerouting connection-mark=ISP3_conn in-interface=ether5-LAN new-routing-mark=ISP3_table passthrough=yes

add action=mark-routing chain=output connection-mark=ISP1_conn new-routing-mark=ISP1_table passthrough=yes
add action=mark-routing chain=output connection-mark=ISP2_conn new-routing-mark=ISP2_table passthrough=yes
add action=mark-routing chain=output connection-mark=ISP3_conn new-routing-mark=ISP3_table passthrough=yes

add action=mark-connection chain=prerouting connection-mark=no-mark in-interface=ether2-ISP1 new-connection-mark=ISP1_conn passthrough=yes
add action=mark-connection chain=prerouting connection-mark=no-mark in-interface=ether3-ISP2 new-connection-mark=ISP2_conn passthrough=yes
add action=mark-connection chain=prerouting connection-mark=no-mark in-interface=ether4-ISP3 new-connection-mark=ISP3_conn passthrough=yes