On a 6128 or 6128Plus with internal memory expansion, "C3" works as expected. The PAL inside the 6128 performs the remapping of &c000 to &4000.
&0000-&3fff: Main ram &0000-&3fff &4000-&7fff: Main ram &c000-&ffff &8000-&bfff: Main ram &8000-&bfff &c000-&ffff: Extra ram page (same ram page selected with 'C7')
On a 464/664/464Plus the result is different because it doesn't have the PAL memory mapping IC (in the case of 464Plus or Costdown 464 the PAL is not activated).
&0000-&3fff: Main ram &0000-&3fff &4000-&7fff: Main ram &4000-&7fff &8000-&bfff: Main ram &8000-&bfff &c000-&ffff: Extra ram page (same ram page selected with 'C7')
C3 looks the same as C1. On fully compatible Dk'tronics ram expansions, A15 is forced to 1 to remap &c000-&4000 but the Symbiface 2 doesn't do this so this is why it works differently on a 464 compared to a 6128.