I have three website on same data base with same users but getting problem….

Hi:
Today i have got it from google search…thanks for this plugin..i hope i will work for me…
let me describe the situation
i have 3 websites
mysite.com
help.mysite.com
newsite.com
currently i am usesing same database and wp_users for mysite.com and help.mysite.com
both are sharing cookies properly there is no problem on both site for login syc.
now i have new domain newsite.com and i have installed it on same database and users. this step has been done. but when i am trying sharing cookies like as i was used for help.mysite.com but its not syc.. even after it i am unable to login on newsite.com

Now i have got this plugin…does it work for cookies syc when i am using already same users on all 3 sites. or it will override the already user sharing on the sites….i am confuse..please help me…
thank you