For such a simple case, use a single simple queue. Multiple interfaces can be specified as targets. There you can set your max up/downloads and your queue types.
It is also possible to do what you want with queue trees, but simple queues are specifically for these basic applications.
Only to understand, you have three devices connected to ports 1-2-3 of the switch that should connect to the internet BUT be separate from devices connected to the other ports of the router? (those, once named properly, are ether2, ether3 and ether4, the switch Is connected to ether5)
In Mikrotik, first port Is ether1, not eth0.
If you want proper isolation then you will have two interfaces (regardless of whether you go with vlans or two bridges.) In this case the download queues have to be separate. I don't know what sort of traffic patterns you're looking at, so this might range from not being elegant to simply not doing its job.
If you are committed to fasttrack and proper queueing, then probably your only path is to forego proper isolation and use port isolation on the switch chip to achieve limited isolation.
Except for some special cases, fasttrack is incompatible with proper queueing...
Does your switch have vlan support? If yes, use the cable to it as vlan trunk with everything tagged and configure the other switch ports as access ports. On mikrotik, configure vlan interfaces on top of the bridge, configure vlans in bridge settings and set everything as tagged on the trunk port. Make sure to use safe mode before enabling vlan filtering to get config roll back in case you lock yourself out.
You can leave the queue setting unchanged, as a queue on the bridge itself will apply to all its vlans together.
If your switch does not have vlan support, you cannot separate the connected devices properly.
Well, it seems to me like you do not want it bridged at all.
We need to define some IP address ranges to better understand your wanted setup.
First let's see what happens without any VLAN.
Let's say that ether1 is directly connected to an ISP router with address 192.168.1.1/24 and thus has IP address 192.168.1.2/24.
Let's say that the bridge comprising ether2-4 is on the network 192.168.88.0/24 and has address 192.168.88.1/24 <- this is the default Mikrotik LAN setting.
Ether 5 (and the switch connected to it, and the devices connected to the switch) are on an alltogether different network 10.10.10.0/24, the ether5 has IP address 10.10.10.1/24 and the switch has (if any) an IP address of 10.10.10.2/24, devices connected to it will have addresses 10.10.10.3-254/24.
The bridge is categorized as LAN.
Ether5 is also categorized as LAN as well.
Ether1 is categorized as WAN.
There is a route for 0.0.0.0 with gateway 192.168.1.1.