Código DHTML:
Ver original<form name="checks">
<input type="checkbox" name="c1" onClick="if(this.checked){document.checks.c1.disabled=false;document.checks.c2.disabled=true;document.checks.c3.disabled=true;}else{document.checks.c1.disabled=false;document.checks.c2.disabled=false;document.checks.c3.disabled=false;}" />
<input type="checkbox" name="c2" onClick="if(this.checked){document.checks.c1.disabled=true;document.checks.c2.disabled=false;document.checks.c3.disabled=true;}else{document.checks.c1.disabled=false;document.checks.c2.disabled=false;document.checks.c3.disabled=false;}" />
<input type="checkbox" name="c3" onClick="if(this.checked){document.checks.c1.disabled=true;document.checks.c2.disabled=true;document.checks.c3.disabled=false;}else{document.checks.c1.disabled=false;document.checks.c2.disabled=false;document.checks.c3.disabled=false;}" />
</form>