There is probably a better way, but this should work.
var loops = [1,2,3];
$.each(loops, function(index, val) {
$('tr').html('<td id=myCell"' + index + '"></td>');
});
This should also work (regular JS):
var i;
for(i=0; i<3; i++) {
$('tr').html('<td id=myCell"' + i + '"></td>');
}
Note how i prefixed id with the word 'myCell', to ensure XHTML compliancy. (thanks to @Peter Ajtai for pointing that out).
EDIT
I just noticed another problem - you're using the .html function to add the cells. But .html replaces the entire html of the matched element. So you'll only ever end up with the last cell. :)
You're probably looking for the .append function:
$('tr').append('<td id=myCell"' + i + '"></td>');