How can I create a .tin file from a ArcGIS TIN file?
I can't seem to find a export to .tin solution.
ArcGIS TIN file is a proprietary file format that cannot be exported to other 3D TIN formats. At least not easily. In the past (up until version 9.x) there was Google extension that allowed to export TIN and other vector data to SKetchUp (then owned by Google). From SketchUp Pro you can then export to a other formats. If you are using ArcGIS 9.x and can find this old extension you can still do this.
Alternatively, you can convert TIN to Multipatch features and Multipatch to Collada (.dea) which "with luck" you should be able to read into any 3D modelling software that supports Collada. Let's say that it works best when source TIN is quite simple.
One other problem is that spatial reference information is not preserved so even if you manage to get your TIN out of ArcGIS and you will need to manually georeference it. (One method is to have a series of 3D control points in ArcGIS and the target software then use those to reference the exported model/TIN)
It is possible but cumbersome. IMHO ESRI is doing everything it can to make this process a one way street. Because ArcGIS/ArcScene is basically a 3D viewer with nearly no true 3D modelling and editing capabilities other than outputs from geoprocessing, the whole workflow is then basically designed to enhance and extend the 3D modelling functionality within ArcGIS by "replacing" existing simple 3D models within ArcGIS with more complex ones; Simly put, you can create a simple cube by extruding a footprint of a house in ArcScene then export that multipatch to Sketchup without georeference info then use sketchup to edit and add detail to that model and then "replace" the initial simple cube in ArcGIS with the enhanced model. So ESRI is utilizing other software to allow to enhance the model within ArcGIS but exporting anything 3D and referenced out of ArcGIS and using 3rd party 3D modelling software is not an option or quite difficult.