There's no hard-and-fast rule. "Three years" is a rule of thumb - HOWEVER...even if you show loss for longer than that - if you can show that you are running the business with the intent of making a profit, it's still legit. I've never experienced an auidt with any of my clients over that issue, so I'm not sure what criteria they use.
Some businesses do take more than three year of loss before they start showing a profit. I suppose, in the event, if you can show them why the loss, and that it's normal for your business, you'll be fine. If they deem it a "hobby", they will disallow the losses. It's murky water. As long as your side business is not something like you and your kid doing the beauty pagent circuit, or you pouring money into your racecar and occasionally racing for a purse (both of which almost never show profit, and are considered hobbies)...
Hmmm. Looks like the best answer I can give you is "depends what your business is". However - I do have one client who is a bellydancer. She teaches, performs, etc. But I think she's showed a profit twice in the last 8 years - and by "profit", I mean "technically" (like $30 net). I keep expecting the IRS to knock on her door and at least ask questions - but maybe they have bigger fish to fry. If it's just been 3 years, I wouldn't worry. Keep pluggin' away at it at the work. Intentions are everything.
Oh yes - if you think what you are doing is a hobby - you STILL need to report the income - but you merely only report enough expenses to bring the profit to zero. So you don't pay tax on it (if in fact you made no profit) - but you can't take losses.