I've tapped many a holes in my time (get your mind out of the gutter OCN) and you should be fine with just going into the M3 holes. If the steel were a lower gauge(thicker) I would say you would have to redrill. If you do redrill the holes be sure to put a stop on your bit so it doesn't grab the metal and send it through the tubes. That would be bad. Be sure to tap by hand and take your time going in and out so you don't hit the tubes. The first part of the tap should be tapered. To be honest, you could probably tap the holes with just the screw itself, but to be sure I would use a tap. The taps don't go bad as long as you use it right and I am sure you would use it in the future. I have a kit that has multiple size as that was the cheaper option for me at the time( commercial door installer).