Timelines for following up after interviews depends hugely on the company. A small company may get together after you leave, chat for half an hour, and make you a job offer the next day. A big company may take a week to gather opinions from everyone who interviewed you, and then have to send them all to HR, who will put you on the list of people they are gong to call back when they've finished the others. Some companies [cough]Google[/cough] will send the results to a committee who evaluate your performance, and may take weeks.
What is normal is that a good company (small or large) will tell you the timescale on which they are going to get back to you. If they said they will get back to you tomorrow, and they haven't, it's perfectly OK to give a polite reminder. If they said it will be a week, never bug them before that. The exception is if your circumstances change, and especially if another company is about to make an offer. Getting competing offers is always a good thing, and it's only fair to let a company know if they have a time limit.
If they didn't give you a time, three days is probably too soon to be bugging them. However what you can do is follow up with a 'thank you' and a polite enquiry. "Thanks for meeting with me on Friday. I really enjoyed talking to you quys. I wondered when I might expect to hear a decision about the next step from you?"
I wouldn't necessarily read anything about whether you got the job into a delay. With some companies, especially big ones, a delay is a good sign, since the next step involves doing something, like setting up more interviews, whereas if you were rejected they just have to call you. Or it might mean they have invited several candidates for interview and want to talk to them all before making a decision. Or it might be that the HR guy is sick, or that one of the interviewers went on vacation without giving his feedback. Stuff happens.