How you open a port mainly depends on your firewall filter. The default action of the filter is to "accept" since it's a router first, so if you aren't blocking anything in the forward chain of the filter, it will be able to pass through the filter just fine. Then all your router needs to know is how you want to NAT that connection to reach whatever server you want, and in your case it needs to remember what interface something came in on so it can route it out the correct one again (that is where mangle comes into play).
/ip firewall nat
add action=dst-nat chain=dstnat in-interface=WAN1 protocol=tcp dst-port=3389 to-address=SERVER_YOU_ARE_FORWARDING_TO to-ports=3389
If you have set up your firewall filter, then you may need to add in an explicit accept rule above your drop rule.
/ip firewall filter
add action=accept chain=forward in-interface=WAN1 protocol=tcp dst-port=3389
If you are only concerned about forwarding over WAN1, then you only need rules for WAN1, however if you want to allow forwarding like that over multiple interfaces then you'll want it for all 3 WAN interfaces. Don't forget the rule to mark those connections for routing, and be sure to have the appropriate route in your routing table. You'll also want a "unique" connection and routing mark for each WAN interface in your case.
/ip firewall mangle
add action=mark-connection chain=forward connection-state=new disabled=no in-interface=WAN1 new-connection-mark=WAN1
add action=mark-routing chain=prerouting connection-mark=WAN1 new-routing-mark=WAN1 disabled=no passthrough=no
add dst-address=0.0.0.0/0 gateway=YOUR_GATEWAY routing-mark=WAN1
Remember, order of firewall rules is VERY important. They are processed in the order that they appear within the same chain. So if you're more specific rule isn't being hit, there may be a more general rule above it that is applying to it first. This document will give you a very good idea of the order of operations and the life of a packet in a MikroTik router.
Also how general or specific your rules are important. If you make the rules too general it will apply to more than what you want and it can break other things, if they are too specific then they will not do everything you are expecting them to. So pay attention to what you are doing when adding in rules, use safe mode in case you break something, and understand the rules, how they work, and what they do.