Both .3ds and .t3d files are used to store 3D content, from 3ds Max and Unreal Engine respectively. The most likely case of 3ds to t3d conversion is related to searches how to best reuse content from 3ds Max for making models for Unreal engine, often for the purpose of game modding.
it is unlikely that you will find simple converter for this. What is more likely is that you can achieve this with knowledge and use of professional 3D modeling tool that offers export to formats accepted in Unreal engine.