I'm not entirely sure what you are implying here?
If you have three external interfaces on the firewall and you don't want to utilise ISP redundancy as you want to switch ISP manually when need all you need to do is simply switch the default gateway when needed?
Have a small txt-file with your changes, let's call it changeroute.txt
set static-route default nexthop gateway address OLD-DEF-GW-IP off
set static-route default nexthop gateway address NEW-DEF-GW-IP on
In expert mode run "dos2unix changeroute.txt" just to make sure we don't have any formatting issues on the file.
When you want to switch you simply run this in expert mode:
clish -c 'lock database override'
clish -f changeroute.txt -i -s
This will write your two commands into gaia and save it, deleting your old default route and adding the new one.
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