Holdem Manager 3, unlike HM2, does support Play Money hands as long as the site write's hand history files and they are in the same format as their real money hand history files. Any unsupported or incomplete hand history files that do not get imported will not get archive which could cause performance problems with the importer and HUD. To manually archive these files if you receive an alert:
Open the Tools > Site Settings menu.
Note the locations of all your configured Auto Import Folders for each site/network.
Select each Auto Import Folder one by one with your left mouse button then click the Open Folder (...) button to open the folder location in Windows File Explorer.
If there are any stuck/unsupported files here you should move (cut/paste) the hands to your C:\HM3Archive folder.
If supported hand history files are not being archived it indicates your Tools > Settings > Import > Archive directory setting may not be properly configured so please check to make sure that is configured.