Why does mail crash when I type more than two letters in the search box?
When searching, mail crashes when I type in two or more letters
It crashes immediately when I type three letters - e.g. searching for Hotel, it crashes when I get to hot....
Mail may be crashing attempting to render a message whose content it cannot decipher. That is more likely to occur on "old" versions of Mail than it is on those incorporated in newer macOS versions.
To update macOS please read Update macOS on Mac - Apple Support.
If you are already using the latest version of macOS that can be installed on your Mac, try rebuilding Mail's message database by following these instructions: Rebuild mailboxes in Mail on Mac - Apple Support. You can rebuild any Mailbox including Inbox, Trash, Junk, etc. I recommend rebuilding all of them.