You could draw shapes in 2d to look 3d, e.g. use a gradient on the shaft to make it look round and draw each side of the spearpoint with shading.  For fancy 3d w/2d you could dynamically adjust the shading when the spear is moved around on the screen.  This probably isn't what you are asking tho.  I don't understand what you mean when you say you have a start and end point joined between them. 

You might want to look at the geomerative or shapes3d libraries for true 3d.

No, I still don't know what you mean .... I think you are trying to do something like the following but it still doesn't look like an arrow.  Maybe it would help to set the z values to 0 initially, draw the arrow shape and make sure it looks like an arrow, then start playing around with the z values?

1. Arrow arrow;
2. Arrow another;
3. void setup() {
4.   size(400,400,P3D);
5.   arrow = new Arrow(80,55,10,175,65,45);
6.   another = new Arrow(90,100,10,175,150,200);
7. }
8. void draw() {
9.   arrow.drawarrow();
10.   another.drawarrow();
11. }
12. public class Arrow {
13.   float cc_x1, cc_y1, cc_z1, cc_x2, cc_y2, cc_z2;
14.  
15.   public Arrow (float x1, float y1,float z1, float x2, float y2,float z2) {
16.     cc_x1 = x1;
17.     cc_y1 = y1;
18.     cc_z1 = z1;
19.     cc_x2 = x2;
20.     cc_y2 = y2;
21.     cc_z2 = z2;
22.   }
23.  
24.   public void drawarrow() {
25.    stroke(0,0,0);
26.    line(cc_x1,cc_y1,cc_z1,cc_x2,cc_y2,cc_z2);
27.   
28.   pushMatrix();
29.  
30.     translate(cc_x2, cc_y2);
31.     float a = atan2(cc_x1-cc_x2, cc_y2-cc_y1);
32.     stroke(173,139,255);
33.     rotate(a);  
34.     line(0, 0,cc_z1, -0.25, -0.25,cc_z2);
35.     line(0, 0,cc_z1, 0.25, -0.25,cc_z2);
36.     popMatrix();
37.   }
38. }

Thank you very much, it was so helpfull. Thanks my friend