Can I shared the files/folder of my time capsule with Mac/PC/iPad outside my lan?

Mac yes.. use BTMM and iCloud or direct AFP connection to the TC.

There is huge amount of info.. just look up remote access time capsule.

PC no.. there is no AFP protocol application that I know of.

iPad no,.. again it is not an AFP protocol device. The only open access method apple allowed is BTMM which doesn't work for ipad.

If you want access for PC and ipad you need to replace the current modem router combo.. whatever it is with one that includes vpn server endpoints.. these can be fairly cheap.. plenty of products with 3rd party firmware will work. VPN is far superior method.