I just tested this... Here's the specifics:
ROUTER A: RB1100AH ROSv.5.12 10.0.0.1/30
ROUTER B: RB1100AH ROSv.5.12 10.0.0.2/30
I have ether 1-3 bonded using 802.3ad with ARP for link monitoring Layer2+3 TxHashPolicy LACP rate is 30s. Same settings on both routers. IP address is assigned to the bonded interface.
Max RAW throughput I'm seeing is 999Mbps with three gigabit interfaces bonded. Throughput doesn't change as I disconnect reconnect the interfaces. This shows failover is working properly but it's not providing additional throughput.
Either that or the bandwidth test utility on RouterOS is capped to 1Gbps. I could try forcing 100Mbps on the interfaces to see if aggregate throughput is accomplished if that's the case.