Count the number of checkbox checked and submit values

function check(){
count = 0;
str = '';
for(x=0; x3){
alert("You can choose a maximum of 3");
return false;
}
else {
alert("You chose " + count + ": " + str.substring(0,str.length-1));
document.form1.submit();
}

<form NAME="form1" id="form1" method="post" onsubmit="return check()">
<p>
<input type="checkbox" NAME="checkbox[]" value="red" />
Red<br />
<input type="checkbox" NAME="checkbox[]" value="green" />
Green<br />
<input type="checkbox" NAME="checkbox[]" value="blue" />
Blue<br />
<input type="checkbox" NAME="checkbox[]" value="black" />
Black<br />
<input type="checkbox" NAME="checkbox[]" value="white" />
White<br />
<input type="checkbox" NAME="checkbox[]" value="yellow" />
Yellow<br />
<input type="checkbox" NAME="checkbox[]" value="purple" />
Purple
p>
<p>
<input type="submit" NAME="Submit" value="Click Me" />
p>
form>