If printed from plastic the best solution for making them eco-friendly is to use the most sustainable plastic. One of the best plastics for 3D printing which is sustainable is called PLA, it is a very simple plastic to use and can be made into lots of different shapes and in lots of different colours, or even colourless and transparent. PLA is made from starch, like from potatoes or corn, is biodegradable and recyclable and can even be composted.
If you are using metal 3D printing, a recyclable metal like titanium, steel or aluminium would be ideal. Steel would be best if it is strong enough for your application and is the easiest and least damaging to the environment to recycle. Aluminium is also good if the part needs to be light. Titanium is one of the most common 3D printing metals but is very expensive and takes a lot of energy to recycle, much more than aluminium and steel because it needs to be melted first. This requires more energy than the other metals because it melts at a higher temperature and takes more energy.
You can use free design software online for preparing designs for 3D printing. Tinkercad is a really nice free software for designing parts for 3D printing online, you just need a browser to use it. Just set up an account and you can start designing parts for 3D printing in no time. If you know more advanced computer design software like Autocad you could use this too. You can design your part first, make what’s called an .stl file and then slice it for 3D printing. The most efficient 3D splicing methods are those which have the least amount of “infill” or “density” to maintain the required shape and strength of the part. This way it will use the least amount of material and also take the least amount of time to build with 3D printing. The splicing software depends on the printer you’re using but Slic3r and Cura are very commonly used for this.