Solved for my case.
It looks like C3 now looks for the <image> tag in the .tmx file, even if it may not be used right now? To add this, I clicked on 'Embed Tileset' in Tiled (small icon below the tileset window/tab in Tiled) and exported to *.tmx again. It now works to import *.tmx to C3 with this change. I imagine the <image> tag is not being used, because the C3 UI requires that you also load the tileset *.png image along with the *.tmx file when loading into a Tilemap.
Here's the .tmx file with 'Embed Tileset' active in Tiled, note the <image source = ...> tag is now included. My previous *.tmx version without 'Embed Tileset' active did not include the <image> tag.
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE map SYSTEM "http://mapeditor.org/dtd/1.0/map.dtd">
<map version="1.0" tiledversion="1.0.3" orientation="orthogonal" renderorder="right-down" width="24" height="40" tilewidth="32" tileheight="32" nextobjectid="1">
<tileset firstgid="1" name="GrassTilemap" tilewidth="32" tileheight="32" tilecount="48" columns="4">
<image source="../Tilemaps/EpicGrassTileMap/GrassTilemap.png" width="128" height="384"/>
</tileset>
<layer name="Tile Layer 1" width="24" height="40">
<data encoding="base64" compression="zlib">
eJztzjEKwCAMQFFdPISbS8FawfufrhEUOihViNv/8BCERI0hIiIiIo2iuBX3BHG1s8rCbs5XSTyf+75nJvwYzZeFvZ2fWJnd4ZrRveYb2v8GcM4L9kEGOg==
</data>
</layer>
</map>