Contact form 7

[checkbox* SelectTree class:SelectTree "Flowering Dogwood" "Red Maple" "Sugar Maple" "Service Berry (Amelanchier Canadensis)" "Service Berry (Amelanchier arborea)" "River Birch" "Elm (Canopy tree)" "Fruitless Sweet Gum (Canopy tree)" "American Linden (Canopy tree)" "American Hornbean (Canopy tree)" "American Hazelhut (Fruit tree)" "Apples (Fruit tree)" "BlueBerry (Fruit tree)" "Pears (Fruit tree)"]

Jquery (Limit select 3)

jQuery(function(){
var max = 3;
var checkboxes = jQuery('.SelectTree input[type="checkbox"]');

checkboxes.change(function(){
var current = checkboxes.filter(':checked').length;
checkboxes.filter(':not(:checked)').prop('disabled', current >= max);
});
});