Roof Truss supported by 3 walls?

Nothing,provided its engineered that way.
Roof trusses are designed with cambers and
other characteristics for specific loadings etc.
eg.you couldn't put a tile roof on a truss system
designed for iron,and visa versa because they
are designed for a particular loading.
another factor taken into account is the ceiling lining,
the pitch of the roof, and the wind zoning,so when it all
complete you end up with a flat ,level ceiling and a
roof true in plane.