You should be able to use PCC to mark each connection in turn, and then add multiple dstnat rules, each using a connection mark.
The per-connection-classifier keeps each client talking to the same server, but you can choose different methods.
Note that this does not test to make sure that the servers are up !
I've not tested the example here, but is should give you the idea.
First mark the connections
/ip firewall mangle add chain=prerouting dst-port=80 action=mark-connection \
new-connection-mark=1st_conn per-connection-classifier=src-address:3/0
/ip firewall mangle add chain=prerouting dst-port=80 action=mark-connection \
new-connection-mark=2nd_conn per-connection-classifier=src-address:3/1
/ip firewall mangle add chain=prerouting dst-port=80 action=mark-connection \
new-connection-mark=3rd_conn per-connection-classifier=src-address:3/2
Then use separate NAT rules, assuming your servers are on the 3 internal addresses
/ip firewall nat
add chain=dstnat connection-mark=1st_conn action dst-nat dst-address=192.168.88.10
add chain=dstnat connection-mark=2nd_conn action dst-nat dst-address=192.168.88.20
add chain=dstnat connection-mark=3rd_conn action dst-nat dst-address=192.168.88.30
Nick