i think it is impossible to make 3 accounts in one ip address ...one ip address will accept only one account
On my case, i can't make 3 accounts. Don't know why.
I have not done that, but I think can not create 3 accounts with 1 IP, but can login 3 accounts on 1 IP. I'm not sure about it.
Today I tried to make the second account, but I got a message from technical support, that I can't post from it, cause I used one IP-address.
The "forum" has probably flagged your IP as "evil", because it thinks you are a bot, or have been banned before etc. I believe you need to pay a small fee to register any more accounts using that IP adress.
Also Registering 3 accounts =/= Using 3 accounts. I could see why the forum doesn't want people to create 100s of accounts without any repercussions..
Explanation about units of "Evil"
https://bitcointalk.org/index.php?topic=711968.msg8044683#msg8044683