This problem has nothing to do with your PC. It has to do with problems along the route taken by the e-mails. When 'server A' sends an e-mail to 'server B', server B sends a 'receipt' back to server A to acknowledge that the mail has got through. If no receipt is received, server A tries again ... and again ... and again...
Eventually, if no receipt is received. Server A gives up trying and returns a 'bounced mail' message to the person who sent the mail. (Depending upon how the its software is configured, server A might make a handful of attempts or several hundred, before it stops trying to send the mail).
The system works fine unless, for some reason, the mail gets through but the receipt doesn't. Then a second copy of the mail is sent. If no receipt is received, a third copy of the mail is sent .... then a fourth .... and a fifth .... etc.
There are many servers along the paths that your incoming e-mails take, so there are plenty of opportunities for things to go wrong, resulting in multiple copies of e-mails being received. (I've had 300 copies of the same message before now).
If you're getting three copies of all your mail (irrespective of the sender or path taken), this suggests that the final server in the path (i.e. the one owned by your ISP or e-mail provider) which is at fault. It seems to be waiting until 3 copies of any incoming mail have been received before sending a receipt.
If the problem doesn't disappear soon, contact your e-mail provider to ensure that they're aware of the problem.
Chris