By dat's description I am assuming that the Cards4Magic punch is a 5mm. I agree with him that the vast majority of specs will not notice the difference IF two or more of the corners are being trimmed. And if you are only trimming two, they need to be on the same short side to not look suspicious. Just trimming one corner makes it stand out like a sore thumb.
I went the Dremel route and failed MISERABLY. I tried various nail files and buffers and had some success but had problems with the card separating. I have gone the curved scissor and nail clipper route and had great success but I am now looking at modifying hundreds of cards.
I searched all over the place and was unable to find a cheap and simple corner punch in the 3mm size. The smallest that I could find was 5mm. I took my search beyond the scrapbooking hobby realm and found big and bulky hand held units that started around $35 +S/H USD and went up from there. I put the project on the back burner for a while. Today was the day to bite the bullet and buy one. I did one last search... and... GOOD NEWS! There's one on eBay from a UK seller. Even though I need it shipped across the pond it is still significantly cheaper than the alternative.
LINK:
http://cgi.ebay.com/Corner-Punch-Mini-3mm-radius-cut-size-NIP-/220645338893?pt=UK_Crafts_Cardmaking_Scrapbooking_DieCutting_Punches_EH&hash=item335f7caf0d
From the limited research that I did, it appears that the manufacturer is European so this may be an item that a local Office Supply or hobby store readily carries, I don't know.
Here is the back up plan should the cheapie above not work. This may or may not be the lowest price out there as I haven't further shopped it. Just an example for reference:
http://cgi.ebay.com/HeavyDuty-PVC-Card-Paper-Corner-Rounding-Rounder-Cutter-/280556731714?pt=LH_DefaultDomain_0&hash=item41527bfd42
Contrary to his description, you'll want the R3 1/8" 3mm rounder- not the R5.
ETA: Better yet, if you go with the bulky unit, you can purchase it from Lybrary.com and give your money to a Magic dealer rather than some guy on eBay:
http://www.lybrary.com/corner-rounder-p-33513.html