I can't comment specifically on the M3 -- but basicall you remove the door panel
-- not a big deal; just do it slowly.

I bought my replacement speakers from BMW -- When one of hte rear speakers
failed, I replaced both so they'd be a matched set -- I"m glad I did -- the new
ones had a distinct different sound to them.

The BMW dealer charged me about $50 a pop (regularly 90 or so)

In the BMW club mag there's an ad for an outfit that touts selling "better" and
"plug 'n play" replacements for bmw....

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I'd go down the local car hifi shop and buy yourself some quality
speakers. They'll have a list of what speakers will fit where, just
say BMW E36 front door.

Although having said that, my E36 front door only has tweeters, the
big front speakers are in the kick panels in front and under the dash.

Dodgy.

If you mean the smaller of the two door speakers, yes they're easy. I
fixed both of mine about a year ago, and posted it here;. Still no
buzzing one year on.

pulled the passenger side front mid range speaker today, the buzzing
one.
There are 11 clips holding the door panel on. What the Bentley service
manual doesn't tell you is, next to the airbag, is a small (< 1 inch)
panel with "AIRBAG" written on it. This is actually a screw cover;
behind it is a fourth Torx screw.

FYI the part no. of the speaker is 65.13 - 8 366 466, spec 20W 3ohm.
diameter is 2.5" (inches). couldn't find a direct replacement from any
local car audio places.

The speaker comes out of its plastic housing pretty easy, with a small
screwdriver and some care. The foam cone material was cracked all round

the outer edge, so I spread a little clear silicone all round the
crack, let it set 30 minutes, and bingo, no more buzzing. Sounds good,
I hope it lasts.