Fijate si esto te sirve:
Código PHP:
<html>
<head>
<title>Loto Test</title>
<p align="center"><b>Cambia celda de color al seleccionar checkbox, y limita a 3 las selecciones</b></p>
<style type=text/css>
body {color:blue;background-color:green;}
.ts {color:hotpink;background-color:white;}
</style>
<script>
function pintar(e){
e=e || window.event;
o= e.srcElement || e.target;
colores=['pink','white'];
color=o.checked?colores[0]:colores[1];
au=document.getElementsByTagName('input');
for(i=0,j=0;i<au.length;i++){
if(au[i].type!='checkbox')continue;
if(au[i].checked==true)j++;
if(j>3){
alert('no más de 3');
o.checked=false;
return;
}
}
o.parentNode.style.backgroundColor=color;
}
function evaluar(obj){
obj.onclick=function(event){
pintar(event);
}
}
window.onload=function(){
au=document.getElementsByTagName('input');
for(i=0;i<au.length;i++){
if(au[i].type!='checkbox')continue;
evaluar(au[i])
}
}
</script>
</head>
<body style="color: #000000; background-color: #FFFFFF">
Por favor, seleccione 3 opciones como máximo:<br>
<form name=exf1>
<table border=1 cellpadding=3 cellspacing=3 align=center>
<tr><td class=ts id=t1>01<input type=checkbox name="c1" value="ON">
<td class=ts id=t2>02
<input type=checkbox name="c2" value="ON">
<td class=ts id=t3>03
<input type=checkbox name="c3" value="ON">
<td class=ts id=t4>04
<input type=checkbox name="c4" value="ON">
<td class=ts id=t5>05
<input type=checkbox name="c5" value="ON">
<td class=ts id=t1>
<td class=ts id=t1></tr>
<tr>
<td class=ts id=t2>
<td class=ts id=t2>
<td class=ts id=t2>
<td class=ts id=t2>
<td class=ts id=t2>
<td class=ts id=t2>
<td class=ts id=t2></tr>
<tr>
<td class=ts id=t3>
<td class=ts id=t3>
<td class=ts id=t3>
<td class=ts id=t3>
<td class=ts id=t3>
<td class=ts id=t3>
<td class=ts id=t3></tr>
<tr>
<td class=ts id=t4>
<td class=ts id=t4>
<td class=ts id=t4>
<td class=ts id=t4>
<td class=ts id=t4>
<td class=ts id=t4>
<td class=ts id=t4></tr>
<tr>
<td class=ts id=t5>
<td class=ts id=t5>
<td class=ts id=t5>
<td class=ts id=t5>
<td class=ts id=t5>
<td class=ts id=t5>
<td class=ts id=t5></tr>
<tr>
<td class=ts id=t6>
<td class=ts id=t6>
<td class=ts id=t6>
<td class=ts id=t6>
<td class=ts id=t6>
<td class=ts id=t6>
<td class=ts id=t6></tr>
<tr>
<td class=ts id=t7>
<td class=ts id=t7>
<td class=ts id=t7>
<td class=ts id=t7>
<td class=ts id=t7>
<td class=ts id=t7>
<td class=ts id=t7></tr>
</table>
</form>
</body>
</html>