They would not know whether you should or should not, they are not experts in using the platform itself.
Its up to you. Having them all in one solution is fine, no matter how often you update the flow etc as it has no impact on the site what so over as it will only update the stuff that has changed.
If its re-usable across multiple other projects, then it should be in a base solution to be shared, otherwise together is fine, but not because of any other reason than, I have no idea if its shared anywhere else.
Another thing to take into account, is not so much how often things change, but how many different people you have working on it. Everyone can create patches and work on the exact same solution at the same time, so its not a big deal to have all 3 of these together. separating them does not make sense to me and it doesn't matter how often those 2 things change and that the Site does not.
But these are limited click down details, but rate of change is not something I use 999.9999999% of the time. Sharability and re-use is.
If these are all dedicated to exactly the same thing.. then yeah I would put them together.
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Thank you!
Sincerely, Michael Gernaey