static (inside,outside) tcp 3389 3389 netmask 255.255.255.255
static (inside,outside) tcp 3388 3389 netmask 255.255.255.255
static (inside,outside) tcp 3387 3389 netmask 255.255.255.255
Where ouside_ip - external address and port for external connections, inside_ip_X - internal servers which provide terminal services.
To allow incoming connections:
object-group service rdp tcp
port-object eq 3389
port-object eq 3388
port-object eq 3387
access-list outside_in extended permit tcp any object-group rdp