The routing itself (gateway=10.0.0.1%pppoe-outX) is fine but as PPPoE is a point-to-point interface, the interface name alone is actually sufficient as route’s gateway (the ip.add.re.ss%interface-name syntax is necessary only for point-to-multipoint interfaces). However, if you want your port-forwarding (dst-nat) to LAN servers to work, you need to assign a connection mark to each connection coming to the 'Tik from the internet to one of the pppoe-outX:
/ip firewall mangle
add chain=prerouting action=mark-connection new-connection-mark=wan1 in-interface=pppoe-out1 passthrough=yes
add chain=prerouting action=mark-connection new-connection-mark=wan2 in-interface=pppoe-out2 passthrough=yes
add chain=prerouting action=mark-connection new-connection-mark=wan3 in-interface=pppoe-out3 passthrough=yes
To route response packets of these connections out via the correct pppoe-outX, you have to translate the connection-marks for packets forwarded from LAN to the internet into routing-marks:
/ip firewall mangle
add chain=prerouting action=mark-routing connection-mark=wan1 in-interface=ether3 new-routing-mark=wan1 passthrough=no
add chain=prerouting action=mark-routing connection-mark=wan2 in-interface=ether3 new-routing-mark=wan2 passthrough=no
add chain=prerouting action=mark-routing connection-mark=wan3 in-interface=ether3 new-routing-mark=wan3 passthrough=no
And you must have the default routes with routing-marks in place:
/ip route
add dst-address=0.0.0.0/0 gateway=pppoe-out1 routing-mark=wan1
add dst-address=0.0.0.0/0 gateway=pppoe-out2 routing-mark=wan2
add dst-address=0.0.0.0/0 gateway=pppoe-out3 routing-mark=wan3
More details here.