there are lots of people which spend soo much time on social networking. so they want to link their several social network accounts. now its an easy task to link all of them.
just open your Instagram profile and go to settings like this
now select option "Linked Accounts" like this
you can see Facebook is already link with Instagram and now if you want to link Twitter just click on the option like this
now your Twitter account will open up like this.
write login details and click on Autjorize App to link it with Instagram.