Fortigate 40c load sharing

Hi

I am currently studying the implementation of Load balacing and Load sharing with Fortinet. Setting up the load balancing is now functional, but not the load sharing. I tried several solutions of the forum but in vain. In my tests, the bandwidth uses only one line. Here is a quick explanation of the architecture

                           [                                     ] PPPOE--------- ISP

Local LAN   --------[            FORTIGATE        ] PPPOE---------  ISP

                            [                                    ] PPPOE---------- ISP

I would like some computers / servers to use only one specific line, and others to use the full bandwidth of the three lines using load sharing. For the stations use the load sharing I would like the load balancing to avoid the maximum bandwidth cutoff The three PPPOE connections are at the same ISP and therefore have the same gateway. We also have several public IP fixed on each connection for our WEB servers, FTP ... For the moment I'm doing the tests on a Fortigate 40c in version 5.2 but if the tests are conclusive I would like to buy a Fortigate 100 at least. Can this project be implemented using a fortigate? Can you refer me so that I can implement load sharing in addition to load balancing please?

Thanks for your help