Sorry, but Easy Profile not have this feature. We add this to our TODO list, is a great feature.

You may add some Javascript to do this.
Example (tested and work) add this javascript code to your template or script file

jQuery(document).ready(function($){

	$('#jform_alias_of_field input').change(function(){

		if($('#jform_alias_of_field input:checked').length==3)

		{

			$('#jform_alias_of_field input:not(:checked)').attr('disabled','disabled');

		}

		else

		{

			$('#jform_alias_of_field input').removeAttr('disabled');

		}

	});

});

if your field have alias "keywords" replace "jform_alias_of_field" with "jform_keywords"

With this when you have set 3 option, the function disable other options

Exactly where i must insert this code? In the master template of my web site?

Yes, you can insert this into some js file of your template (at the end of file).

If you send me your site link I can tell you which file.

You can insert also into our js file /components/com_jsn/assets/js/name.js but if you update easy profile you lost the modification.

There is another problem.
Suppose that during the first compilation of profile, we have select three options. Properly others are disabled. But if you return to the edit profile and add one new option, this to me is granted. I can even select them all and save. How can I avoid this problem?
Thanks
Renzo

ok,
try this code:

jQuery(document).ready(function($){

	$('#jform_alias_of_field input').change(function(){

		if($('#jform_alias_of_field input:checked').length==3)

		{

			$('#jform_alias_of_field input:not(:checked)').parent().css('opacity','0.6');

		

		}

		else if($('#jform_alias_of_field input:checked').length>3)

		{

			$(this).removeAttr('checked');

		}

		else

		{

			$('#jform_alias_of_field input:not(:checked)').parent().css('opacity','1');

		}

	});

	$('#jform_alias_of_field input').change();

});

I have found another problem with chrome HTML5 validation.