вы видимо имеете в виду radiobutton ;-)
global known
set known "no"
radiobutton .r1 -text "I know" -variable known -value "yes"
radiobutton .r2 -text "don`t" -variable known -value "no"
radiobutton .r3 -text "What,what?" -variable known -value "dummy"
где-то так...
почитайте man`ы - полезное занятие ;-)