BIG IP 1600 - Trunk configuration - More than 4 ISP Internet providers

Also, I'm not sure what you are looking to achieve with 3 switches in the infrastructure. If you are looking for redundancy, I may recommend putting two interfaces from each router to the lan, and using some method of unifying the forwarding plane of the switches. Whether that is in the form of stacked switches, or something along the lines of Arista / Cisco Nexus / Cisco VSS or something similar. Otherwise, the third switch in the middle really does not serve much of a purpose in the configuration.