Hi, welcome to the community.
If I understand correctly, you're trying to make a tube weldment which follows a complex 3D trajectory, but is made out simpler segments (2D trajectories, or tubes bent in only a single plane).
Start by making your 3D trajectory. One way would be to create a composite 3D curve by intersecting two 2D sketches. Create two 2D sketches on planes perpendicular to each other then intersect them with the "Intersect" command. This gives you a composite 3D curve which follows the profiles in both base sketches.
I can think of two ways you can take the 3D curve and cut it up into several segments. One way would be to place several datum points on the curve where you want the pipe segments to start/end. Then use the trim command to trim the curve at those datum points. You'll have to flip the trim arrows a couple times to tell it to keep both sides of the trimmed curve. Repeat this for any other pipe segments.
Another method is to trim the curve "on-the-fly". Let's say you're going to use the pipe segments in a sweep command. Start by placing the datum points on the curve as before. In the sweep command, select the 3D trajectory and open the trajectory detail window. In the "Options" tab change the end length adjustments from "Value" to " Trim at Reference" and select two of the datum points. Then create the sweep section as usual and repeat for any other segments. I think this method is preferable.
Your success with this depends on how you created the composite 3D curve. You have to make sure arcs in each of the 2D sketches don't intersect. If they do, your composite 3D curve will have sections where they aren't 2D.
You could also just use Pro/Piping.
