If you use OEM it's a pretty simple process, first you create a profile and then alter the user to use the profile..
You must also set the init.ora parameter to
RESOURCE_LIMIT=TRUE
But to create manually, perform the following. Actually even when using OEM, you can only limit the profile's SESSION_PER_USER to set values. You'll need to alter the profile to adjust the limit manually after creation.
If performing the whole process manually...
CREATE PROFILE TEST LIMIT SESSIONS_PER_USER 3;
ALTER USER FRED PROFILE TEST;
Cheer,
OCP 8i, 9i DBA
Brisbane Australia