Without Power Apps, you could achieve this through default SharePoint list form and alert.
Here're steps:
1.Create a SharePoint list with columns you required. When the user creates a new item, it means that the user submits a form.
2.Go to the newly created list -> Manage my alert -> Add alert -> Choose the newly created list.
(1)Send Alerts To: User A; Change Type: Existing items are modified; Send Alerts for These Changes: Someone else changes an item created by me.
(2)Send Alerts To: User B; Change Type: All changes; Send Alerts for These Changes: Anything changes.
(3)Send Alerts To: User C; Change Type: Existing items are modified; Send Alerts for These Changes: Someone else changes an item.
3.Go to List settings -> Advanced Settings -> Item-level Permissions -> Create and edit access -> Set Create items and edit items that were created by the user. After setting, only the creator can edit items.
4.Users will receive corresponding alerts. Users click the item in the email, then could make comments to the item.
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