> restart:
with(geom3d):
point(A,1, 0, 0):
point(B,0, 1, 0):
point(C,0,0,1):
plane(P, x+y+z=6,[x,y,z]):
point(T,x,y,z):
sol:=solve([distance(T,A)=distance(T,B),
distance(T,A)=distance(T,C),Equation(P)],[x,y,z]);point(T1,rhs(op(1,sol[1])),rhs(op(2,sol[1])),rhs(op(3,sol[1])) ):