I, too, lost my Sent, Trash, and Draft folders. I learned accidentally, however, that if I did a search in email, sent emails turned up in the search results even though I have no visible Sent folder! So I knew the sent emails were there somewhere (my main email is an EarthLink pop3 account)
Trying every suggestion I could find online, at some point, I went to Settings>Passwords&Accounts>Accounts>Gmail. On the Gmail account page, I toggled OFF the Mail.
I then used the suggestion on this thread to send myself an email, then deleted it. Voila! The Trash and Draft folders magically reappeared. I then used another suggestion in this thread to send myself an email, delete it, go into the Trash folder, and move that email back to the Inbox folder. My Sent folder did not magically re-appear. At that point, I happened to go back to Settings>Passwords&Accounts>Accounts>Gmail, and this time on the Gmail account page, I toggled Mail back on.
when I went back to email right after that, the Sent folder magically appeared! Don’t know if moving mail from Trash to Inbox, or turning Gmail off and on, or the combination of doing both brought my Sent folder back, but I thought it might help someone else with this frustrating and serious problem.