Tiptorial: manually reducing the poly from hair (if you're crazy enough and have a lot of free time)
First of all, get rid of the undersides and remap the whole mesh to the proper ts3 hair texture size;
Select one strand, revert the selection with ctrl+i and hide it all with H, to isolate the strand we want to work (do not separate it into another object, if you do that you'll mess with the strands order and will have transparency issues!);
We need to convert the tris into quads, for this, press alt+J. This strand had almost no issues converting it, but some might present it, then you'll have to select each square manually and dissolve the faces to turn it into a quad. It's easier to do that in the UV map with sync selection by selecting it like a checkerboard, then dissolve everything, then select one tri and press shift+g and select polygon sides, this will select everything that was not converted and then you can dissolve it.
After everything is in quads, alt+click on a edge to select the whole edge loop, then dissolve the edge. You can also press G twice to slide the edge along the mesh to better adjust it.
When the strand is like you want it, you can select it and assign to a vertex group so you can easily select all the dissolved strands to better work with the whole mesh (remember, do not separate any strand!)
And now you'll repeat the process with all the other strands 🫠