The message
from "Mary Fisher" contains these words:

This morning I pondered about this matter.

You should have pondered a bit more because you are wrong.

The three-legged stool in milking parlours is more stable than a four
legged
one because it was on an uneven floor. The OP wasn't wanting to stand on a
stool in a mistle.

3 legged stools are found outside of milking parlours these days, not
inside, and uneven floors occur in all sorts of places where they might
be the most convenient thing to hand to stand on.

But the milkmaid - or man - also only *sat* on the stool, it wasn't
used for
reaching high. If it had been the centre of gravity of the combined stool
and body would have been much higher, thus increasing any instability
produced by the stool leaning to one side on the uneven floor.

The combined centre of gravity (whatever you mean by that) is a red
herring. If the stool doesn't fall over on its own accord it should be
possible with care to stand on it. If the floor is so uneven (or even if
not) that the stool needs assistance stick a brick or whatever under the
lower legs, it is easy enough to get a 3 legged object approximately
upright and stable which is more than can be said for a 4 legged object.

In my experience ladders are used in mistles to reach anything higher than
full stretch of the body.

Not ever having been in a working mistle I wouldn't know but my
experience both within and outside a barn is that the nearest thing to
hand is likely to be used for added reach be it bucket, crate, upended
pallet, or whatever, even a set of steps. But a ladder? Not when
anything else will do.

And only someone who hadn't thought the matter through would use a three
legged stool to stand on even in a household situation with a nice flat
floor.

I wasn't going to respond until I read the paragraph above but the
absurdity of this statement and the implied insult decided it for me.
Depending on the design of each a 3 legged stool can be safer even on a
level surface than a 4 legged stool. A 3 legged stool of reasonable
height typically has splayed legs so that the horizontal projection of
the seat onto the floor can fall wholly within the perimeter defined by
its 3 feet. Some 4 legged stools with similar sized seats have vertical
legs and thus an unstable overhang on all 4 sides of the seat should one
be foolish enough to stand on it.

We don't use three legged tables and even those oddities of three legged
chairs use different angles where legs attach to the seat, to make up for
their instability. They're not very popular either ...

You might not use a 3 legged table but tables with a centre support do
exist and AFAIK have 3 legs.

Even 3 wheeled cars are (allegedly) stable in normal use.

--
Roger