This seems to be what everybody wants to know so I'm finally gonna tell ya how to do it.
First of all, there are 2 types of 3do's held in the track.3do: infield and outfield. I'll relate outfield first since they are easier.
First, decompile the track.3do into 2 def files and open the flavor.def file. flavor 13's are the position of the 3do, first 6 numbers are those that show up in the 3do edittor. next number just has to have 5 digits, I usually us 65565, last number calls to the flavor 14 that comes right before it.
The flavor 14 defines what the object is, first number is the bite offset, second is usually 0. to find the offset, go to the FIL section and add the object name to the bottom. then determine if the FIL above it is texture or object and sort either flavor 5's (textures) or 14's. Find the highest value and then add the number of characters in the 2nd to last FIL plus one.
Then, the first Flavor 4 after the last flavor 13 calls to the 3do's. Add the id# of the flavor 13 to that flavor 4 line. Add it to the end to show in front of everything and to the begining to show behind everything. You will need to add it to all sections of the 3do where you want it to show up. Sometimes a flavor 13 will not immediately precede the flavor 4 so search for flavor 16's and take the 3rd flavor 4 after it and add your 3do's flavor 13 id# to it.
Infield 3do's follow the same flavor 14 and 13 rules but aren't called to by the first flavor 4 after the set of 13's and 14's. Instead you need to trace them back through the 3do. Go to first flavor 16 and then the first flavor 4 after it. take the 2nd call to in that flavor 4 and search the def file for it. it should show up before the flavor 16 line so it may give you a warning at first, just research it. You should come up with a falvor 6, scroll up to the line right above it, should be a 4. take the first call to and search for it. should come up with a flavor 10. scroll up to the line before it (should ba a 4) and search the first call to. You should get a few more flavor 4's with the number as a call to. Keep going until you get the flavor 4 that has it as an id #. add the flavor 13 id# to that flavor 4's call to's. Then do that for all sections you want the 3do to show up in.