PowerShell replace unknown 3 letter word after operator

Question

I am new to regex, can someone help me figure out how to replace unknown text while reading from a file

I am searching through a file for DEST=XXX

I dont know what XXX could be, I just know it's a 3 letter String

Here what I have so far

   Get-Content -path $filePath -replace 'DEST=\w{3}','DEST=MTG'