var objLeft = document.getElementById('left');
var objRight = document.getElementById('right');
var objContent = document.getElementById('content');

if ( objLeft.offsetHeight < objRight.offsetHeight ) {
	objContent.style.height = objRight.offsetHeight + 'px';
	objLeft.style.height = objRight.offsetHeight + 'px';

} else {
	objContent.style.height = objLeft.offsetHeight + 'px';
	objRight.style.height = objLeft.offsetHeight + 'px';
}

if ( objContent.offsetHeight < '300') {
	objContent.style.height = '300px';
	objLeft.style.height = '300px';
	objRight.style.height = '300px';
}