SD-WAN Sessions

We have recently had a 400F installed with three DIA lines.  We have been set up with an SD-WAN configuration where traffic is spread over all 3 lines.  This seemed to work great and the lines were balanced but we started to notice that certain websites would drop out, logging people off and applications such as Outlook would loose connectivity.

We have narrowed it down to the fact that session traffic which starts on port 1 can then jump to port 2 or 3 changing our outbound IP and thus breaking any session the user had with the website they were visiting.  

We would ideally like to keep the round-robin approach where data is spread over the 3 lines automatically but we're not sure how best to configure the gate so traffic gets around this issue.

Some research is pointing towards having the SD-WAN set up so that all traffic goes to port 1 and then to port 2 on a failure but is there a way to have traffic move to port 2 after a certain utilisation is reached?

Any advice greatly appreciated, I'm very new to Fortigate but slowly finding my way around.

Some options we've found online is to have a round-robin