Dear @Prajay,

I have 3 ISP connected to my TL-R605

Speed as follows

ISP1-100 Mbps

ISP2- 200 Mbps

ISP3- 100 Mbps

So I want LAN out put 400 Mbps for client so which setting should I use in router page under Transmission option>Load Balancing?

TP-Link load balance router could help you make good use of all bandwidth of WAN links to speed up the whole traffic. 

On the Transmission-->Load Balancing page, ensure the Load Balancing is enabled under Basic Settings. you can configure the other two options according to your needs.

(1) Application Optimized Routing

This feature can make applications that run multi-connection choose the initial WAN to avoid the connection lost, suitable for applications like online payment.

(2) Bandwidth Based Balancing Routing on ports,

With this feature, the router will assign multi-session connections to different WAN ports based on the bandwidth ratio of each WAN port.

Note: need to configure the bandwidth(Upstream/Downstream) provided by the ISP under Network -->WAN for each WAN.

The Load Balance feature on the router selects which WAN port is session-based. If the application only needs one session, it will only use one WAN. Read this FAQ to learn how the load balance works. https://www.tp-link.com/support/faq/2079/