Here is another approach, it is more difficult, but does what is required:
{selected="none"}
{formtoggle: name=optiona; default=no; trim=right}
{if: selected == "b"}{optionb=no}{endif: trim=yes}
{if: selected == "c"}{optionc=no}{endif: trim=yes}
{selected="a"; trim=yes}
{endformtoggle: trim=no}
{formtoggle: name=optionb; default=no; trim=right}
{if: selected == "a"}{optiona=no}{endif: trim=yes}
{if: selected == "c"}{optionc=no}{endif: trim=yes}
{selected="b"; trim=yes}
{endformtoggle}
{formtoggle: name=optionc; default=no; trim=right}
{if: selected == "a"}{optiona=no}{endif: trim=yes}
{if: selected == "b"}{optionb=no}{endif: trim=yes}
{selected="c"; trim=yes}
{endformtoggle: trim=no}
{=selected; trim=no}