An equation by nature sets two things as being equal. That lowers the degrees of freedom in the variable set by one. For example, if you have a single dimension where x is any real value, you have one degree of freedom (x can be anything). When you set an equation in it, say 2x + 3 = 5, that reduces the degree of freedom by one, to zero; that is, x = 1. In a plane you have two degrees of freedom, and can reduce that by one by setting an equation there; e.g. y = x^2; that is true for x equals any real value, say a, as long as y equals a^2. In space you have three degrees of freedom but nearly any equation reduces that freedom from three to two, giving you a plane.
To get a solution with three degrees of freedom, you could start with four and limit that by one to three.. say, (x,y,z,t) in a 4D space and use the equation t = 0, which is true for any real values of x, y, and z, as long as t is zero.
alternatively.. you could use a non-conditional equation that does not reduce the degrees of freedom. Say, the values x,y,z for which
x + y + z = x + y + z.