I already solve the problem. I couldn't get the instance but when I save the object first
var thisPicker = jQuery(this);
and on button3 get the instance,
var inst = thisPicker.mobiscroll('getInst');
everything works fine.
Whole code...
// Date mobiscroll picker init
jQuery(".datepicker").each(function (index, element) {
var thisPicker = jQuery(this);
jQuery(this).mobiscroll({
preset: 'date',
theme:'ios',
display : 'bottom',
button3Text:'X',
button3: function(){
var inst = thisPicker.mobiscroll('getInst');
thisPicker.val('');
inst.cancel();
},
});
});