Can I use sharing for my accounts if I already pay for them

If I pay for 3 gold accounts already, can I use the sharing feature to keep those accounts, and only pay for one.

Absolutely!

Before the days of the family offer on accounts you had to pay for each account. With the launch of the One, you specify one account that the console is bound to, setting the home console for that account.

While (digital) games on that console are shared by default when owned by the primary account, you need to manually specify that you want to share the Gold feature with accounts on your home console. You can read more on the topic here!

Note that it works slightly different if you are not on your home console. While you can still enable other accounts to benefit from Gold, the sharing is only temporary as long as the account sharing the subscription is logged in.