Mixing battery sizes
The small cell will run out of juice before the others do.  The two good cells will continue to force current through the dead cell, which will attempt to charge it backward.  The cell will be ruined (not a big deal) and it is very likely to leak (possibly a big deal).  

Until that time, the battery will function.  Also, after the small cell dies, you now have two half-used C-cells to manage.