<?php
$a = [
 'Fieldname1:value;Filedname2:value;Fieldname3:value',
 'Fieldname1:value;Filedname2:value;Fieldname3:value',
 'Fieldname1:value;Filedname2:value;Fieldname3:value',
];
$array = array();
foreach($a as $b){
	$temp_array = array();
	foreach(explode(';',$b) as $c){
		list($key,$val) = explode(':',$c);
		$temp_array[$key] = $val;
	}
	$array[] = $temp_array;
}

print_r($array);