Do you have it weighted correctly? 210 would be correct with a equal split between all wans.EDIT: Sorry I am coming from a different router (still new to opnsense specific's), it does not appear you can adjust the load-balance weights, so I would say your probably reaching your max throughput as such.
Unequal Balancing (Weight)If you have a non symmetric setup with one IPS having a much higher bandwidth than the other then you can set a weight on each gateway to change the load balance. For instance if you have one line of 10 Mbps and one of 20 Mbps then set the weight of the first one to 1 and the second one to 2. This way the second gateway will get twice as many traffic to handle than the first.To do so, go to System %u2023 Gateways %u2023 Single and press the pencil icon next to the Gateway you want to update. The weight is defined under the advanced section.