For each public IP create an address:

/ip address

add address= comment="IP 1" interface=ether1

Then create the NAT for each public IP. First create entries for allowing the traffic:

/ip firewall nat

add action=src-nat chain=srcnat comment="Server 1" src-address= to-addresses=

add action=dst-nat chain=dstnat comment="Server 1" dst-address= to-addresses=

And then you can create the rules for port forwarding:

/ip firewall nat

add chain=dstnat action=dst-nat dst-address= to-addresses= protocol=6 src-port=80 dst-port=80