SOURCE

function limitInput(inputDom){
	inputDom.addEventListener('input',function(){
		var v = this.value+'';
		var i = v.indexOf('.');
		var isZeroFirst = v.indexOf('0') === 0 && i <= -1;
		if(i === 0){
			this.value = '0.';
		}
		if(isZeroFirst){
			this.value = v.substring(1,v.length);
		}
		if(i>0 && v.length - i > 3 || (v.match(/\./g) && v.match(/\./g).length > 1) ){
			this.value = v.substring(0,v.length-1);
		}
	})
}
limitInput(document.getElementById('it'))

<input type="text" id="it"/>
<input type="text" id="it3" oninput="limitInput(this)"/>