plotting a function of three variables



I am still not able to plot a function of three vars, one of them
having been fixed. Let us call t the third variable; here are my last 
two attempts:


--1:

  (C4)  ff(x,y,t) := x*t/(y-3);

					       x t
  (D4)                         ff(x, y, t) := -----
					      y - 3

  (C6)   plot3d(ff(x,y,t),[x,2,3],[y,2,4],[nticks,100]);

  Error: Expected 3 args but received 2 args
  Fast links are on: do (si::use-fast-links nil) for debugging
  Error signalled by MACSYMA-TOP-LEVEL.
  Broken at LAMBDA.  Type :H for Help.
  MAXIMA>>

--2:

  (C7)    plot3d(ff(x,y,t),[x,2,3],[y,2,4],[t,1,1],[nticks,100]);

  Error: The ECASE key value |$t| is illegal.
  Fast links are on: do (si::use-fast-links nil) for debugging
  Error signalled by CATCH.
  Broken at MACSYMA-TOP-LEVEL.  Type :H for Help.



Is the function definition correct? Should i use ':' rather than ':=' ?
Any suggestion to plot f(x,y)?

A further question: if x and y were functions themselves, say x(z)
and y(z), should i declare such dependence in the f definition:

 f(x,y,t) := x(z)*t/(y(z)-3);

Any example about plotting functions and functions of functions will
be appreciated.

thank you 


--
Paolo Pumilia