k well, then do this (assuming you have milkshape3d)
first find the model with the ssj3 hair you want, and then the vegeta/gotenks you want to put it to
go to tools>half life>decompile HL model and locate the ssj3 model and select it.
a box will come up with 4 choices, ( or 3 i forget), reference, sequences,qc file, and textures i think, for this one, just pick reference and textures.
now go to import, and select the half life SMD option, i forget exactly what it says (i haven't had to do this in almost a year) and select the goku models reference file, its probably called goku.smd,
another multi-box should pop up, uncheck the rename bones and the skeleton choices (you dont need the bones for the hair)
now you SHOULD have the goku model right in front of you, now just select and delete everything but the hair.
save it to an ms3d file for safe keeping, we will move on to the next half now
now do everything you did for the goku model to the gotenks/vegeta model, except this time select ALL 4 choices at the decompile screen.
now import the reference smd of that model, anybodys guess what the gotenks one is but vegetas SHOUlD still be vegeta. leave everything checked BUT rename bones, ( i dont know what it does, but i know it works without it, so why risk it?)
now you should have the complete model on your screen, bones and everything. NOW comes the merging of the 2 parts.
go to files then merge and select the model of the hair, so long as it didnt have any bones in it, it SHOULD go right in without problems.
now using the scale and move tools, fit it to the vegeta/gotenks
now you have to assign it to the skeleton, hide all the groups but the hair if you can, it makes the next part alot easier,
select the head bone for now (may require tinkering later) in the bones/joints tab
while holding shift, select the hair and click assign in the bones/joints tab.
now all you got to do is export it over the reference smd file of the model you were trying to edit, and move any required texture files into the same folder and click compile in the half life tools part of milkshape.
credits go to nuttzy for this short tutorial! trunks follow this tutorial an u will be fine!