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any ideas how to do this in the best way??, I'd like to make a fishing game in 3d...

D3D has some 3D line commands which would come in use. Look up the command "D3d_Line3d".

OK, i made a line with d3d_line3d. I started to understand dow its working. Now my problem is to make a dynamic curve line, to change the rays, for instance when I want to launch the rod wire in the water...to be realistic....any suggestions???

Well, you need several 3D lines so that the fishing line can curve etc.

Usually, the most direct method would be to have an array of 3D positions that try and stay within a fixed distance from the previous position in the array. So, the first position would be the end of the rod, then the second position maybe a foot of line. Then each loop, you update the array positions depending on their velocity, then again to constrain them to a fixed distance. Add in gravity to the first calculation and believe it or not - you'll have a reasonable string simulation, especially if it's balanced right.

With those rules, you could add in that any position that's under the water gets it's velocity reduced by a great amount, otherwise the velocity is reduced slowly. With that, it should be possible to tighten and slacken the line just by moving the rod, letting the line sink etc.

I've gotten some nice results with this technique in 2D, I'll see if I can make time to come up with an example.

To do it convincingly will take some knowledge of physics and math. Specifically, as Van B said, you need to create a series of lines that will be connected to each other and make a series of arrays that correspond to the X, Y, and Z coordinates of the points where the lines segments join together.

To determine the values of the array is where the knowledge of physics and math will come into play, as you'll need to calculate the curvature of the line based on how the weight of the line makes it sag. If you're good at math but not so good at physics, you can make the line a simple parabolic curve.