One option only

Snippet Exchange
1

{formtoggle: name=11/26; default=no} {endformtoggle}
{formtoggle: name=11/27; default=no} {endformtoggle}
{formtoggle: name=11/28; default=no} {endformtoggle}

{if: count(filter([`11/26`,`11/27`,`11/28`], item -> item == "yes")) == 0}{error: Please select one option; block=yes}{elseif: count(filter([`11/26`,`11/27`,`11/28`], item -> item == "yes")) > 1}{error: You may only select one option; block=yes}{endif}

3

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}