I moved this to NGFW for better response.
Appreciated and answering the question is more Appreciated
If you wish to use more than 2 interfaces in aggregate NGFW engine version must be in 5.8 version as 5.7 and older versions support only 2 interfaces in aggregate. This 5.8 enhancement is mentioned in NGFW 5.8.2 release notes:
Interface link aggregation
Interface link aggregation has been enhanced to support up to 8 interfaces in load balancing mode.
Feature stability has been enhanced and Link Status test for aggregated interfaces is improved.
Thanks for sharing the info, it worked fine