Hello,I need an opinion before trying something
"dangerous":

what will happen if I install a monster card
connected to a voodoo 3 like if it was
any other normal card?.
(I used to have a s3 trio working with the monster).

The good-ending theory is that it will work
with no side effects. I'll have a bat file
to switch the opengl and glide dlls,or I'll just
place the right ones in the folders of what
I'm planning to run,and for direct3d,
the v3 one will work under the "primary display driver"
option and the monster one under the "monster 3d blabla"
selectable option like it used to with the s3 card.
And,if I'm lucky,the monster acceleration will apply
to the already fast v3 and it will be faster.

The bad-ending theory is that both cards will get burnt :(

Should I try?.

Well, the only advantage you could get out of it, would be the increased compatibility (e.g. if you run Games or Applications that were solely designed to run on the Voodoo1, and fail on the Voodoo3).

Since both cards will always run completely separate from each other, they won't accelerate each other of course.

Whether the added compatibility option (native Voodoo1) is worth the hassle (Glide/OpenGL Switcher Utilities etc.), you'll have to decide depending on your own situation.
-------------------------
Technically, there is no risk involved (e.g. the Cards won't be damaged), but conflicting Driver Files might result in Problems, if the Tools used to split/select between them don't work 100%.

argh,I can't try due of the motherboard layout,
the processor fan and the v3 fan.
The s3 and monster combination fits,
but monster and v3 doesn't.
The s3 one is a smaller card,so it fits in
a pci slot that is just in front of the processor
and its fan,but the monster doesn't.
It used to be in the slot on the left of that one,
but now it doesn't fit there because of the
v3 fan,(with the v3 in the slot that is in front
of the processor).
The third pci slot is not avaible because of
the design of the case,
it's bay/hole/slot/whatever-it's-called
is not in front of the pci slot,so I can't
place a card there.
Anyway,I'll try in the next upgrade,perhaps
the monster and v3 combination fits in another
motherboard (why did they put the processor's socket
in front of a pci slot?)