Ronyfa,
This is possible but awkward (personally, I would bother with the filled triangle and just use the weld symbol, this is more than explanitory)
However,
In drawing, draw a sketched line. Use SNAPPING REFS to pick the two lines on your view that the weld will be between. Complete your triangle (I usually overdraw the third line and use Edit>-Trim>-To Intersection and delete lines that are not part of the triangle. This ensures that the triangle is 'whole'). Then Edit>-Fill>-Solid and choose the whole triangle. (You can change the colour by RMB on the filled section and going to properties). Once you have that all completed select all the weld (Incl. fill) and Edit>-Group>-Relate to View (ensures that ifyou move the view, the weld details go with it)
You can attach the weld symbol to the third line
Kev