i would suggest the RB3011 and plugging the WANs into 1 switch group and LANs into another. Segmentation is easy in mikrotik and so is configuring the network/routing tables. You should know that regardless of any router, they will all select 1 WAN as the main WAN for use of router operations (like DNS). Make sure to unswitch any port first and you wont even need to set up vlans, only set your 3 LAN subnets, DHCP for each, firewall for each and you're set. You can also set up radius that way as well and an AP for multiple SSIDs. you can also use the Rb2011 too as long as it has 2 seperate switch chips or more CPU connected ports.
With mikrotik, just perform layer 1 segmentation (interface and switch management) and layer 3, you dont even need vlans but if you do use 1 AP with 1 cable you will need vlans. Sure there are cheaper routers that will do the job but i would not have LAN and WAN on the same switch even with vlans and layer 1 segmentation using the switch (meaning not switching the ports). The RB3011/2011 can be rackmounted as well and have passive cooling unlike the ERPRO. The RB3011 has a single POE out capable of powering a ubiquiti AP (you should check the voltage and watts match so that enough power can be supplied). Just remember that with POE you want the cables to be as short as possible. You will have lots of free CPU left on mikrotik to run scripts and other functionalities even dude server which is mikrotik's network monitoring. It also has SFP port so that means 1 modem less if a valid SFP module exists for your internet medium (ERPRO has 2 and is also a good consideration with CPU connected ports only, just costs more).
Im not sure which ubiquiti router can do it, probably all if its the same series, just read the description about radius.
So i would suggest either the
RB2011 (make sure it has gigabit ports and 2 switch groups or as many CPU connected ports as WANs at least and has 1 SFP, doesnt support dude server)
RB3011 (has POE out and 1 SFP)
ERPRO (has CPU connected ports only and 2 SFP)
pfsense based mini PC router (the best option if you use only 1 AP and vlans as long as it can perform NAT for 3 different networks, each with their own WAN)
Setting up internet is pretty easy. First set up the connection and make sure the router can connect to internet via all 3 lines, set up 3 vlan interfaces and attach to the ethernet interface, give each vlan interface its own LAN ip (basically it is the gateway IP for each LAN), set up NAT rule for each LAN with the syntax of from 192.168.1.0/24 to 0.0.0.0/0 action masquerade out interface WAN1 (you only need 3 of these rules). despite mikrotik routers with switch chips allow configuring switch chips, it is better to do it on the CPU instead as you can use those vlans in your config. With pfsense and ubiquiti you must select not to load balance to wans or have fail over or any sort of handling with it but you also will need to define 3 LANs as well. With mikrotik defining a network is pretty straight forward as giving an IP address to an interface as both ubiquiti and pfsense ask you a bunch of stuff when you add a network but load balancing and QoS are very difficult to set up on mikrotik but easier to set up on ubiquiti and pfsense.
If you plan on caching such as a transparent proxy cache, the ERPRO is capable of doing it at faster than your WAN speed but pfsense would be a better choice for it as you can attach a hard drive/SSD and use that for cache. Mikrotik can do it too but from my experience, mikrotik is the best router out of the 3 but has the worst software while pfsense has the best software and is a very good router too. ubiquiti is in the middle, its the worst out of the 3 as a router but better than mikrotik when it comes to software (its basically a MIPS based embedded linux machine that lets you install debian compatible packages compiled for MIPS).