Afternoon @Anonymous. You may have to contact Samsung to see if Smartswitch will work on Lumia 640 to Samsung A3 2017. According to the User Manual on my Galaxy S8+ it does say it doesn't work with every phone and it could be it won't work with your Lumia 640.
I also looked to see if there were any instructions for transfers between a Windows phone and a Samsung phone but it only dealt with iPhone to android (Samsung) and android (Samsung) to android (Samsung).
I've used Smartswitch to transfer files from my old Note 4 to my new Galaxy S8+ with no problems. So Samsung may be your best bet to answer your query. Sorry I can't be of more help.
Lover of all things Samsung. Currently using Samsung Galaxy S10 Plus 128gb