.htaccessで振り分け
RewriteEngine On
#Options FollowSymLinks
# 携帯用
RewriteCond %{HTTP_USER_AGENT} (DoCoMo|KDDI|DDIPOKET|UP\.Browser|J-PHONE|Vodafone|SoftBank) [NC]
RewriteRule ^$ http://www.example.com/a/ [L,NS]
#スマートフォン用
RewriteCond %{HTTP_USER_AGENT} (iPhone|iPod|iPad|Android.*Mobile|BlackBerry|Windows.*Phone) [NC]
RewriteRule ^$ http://www.example.com/b/ [L,NS]
#pc
RewriteRule ^$ http://www.example.com/c/ [L,NS]