Just get an outbound load balancing setup. You don't need a RM1000+ load balancer, pick up an an entry level Mikrotik RB750 and setup per connection balancing across the 3x PPPoE interfaces. You can go ahead and manually assign groups of PCs to individual gateways but this would be a waste of bandwidth when you can pool the 3Mbps together and distribute it across your entire network.

3x DSL modems (DSL bridge mode) -> 3x ports on an RB750/any other load balancer -> Your layer 2 switches

I've got this setup with 4x Unifi 20mbps links :



What engtat recommended is manual gateway assignment, you basically setup a flat network.. 3x gateways (IPs .1, .2, .3) in the same subnet with every other computer system. Then you use DHCP leases to assign the .1/.2/.3 gateway IPs to individual PCs based on which department their MAC address is registered to.