Garcia, that is a good question.
Take the example of the OP. I don't know where she is located, but equal revenue every day of the week is not always realistic. Some days will be slow, others busier. Will the average still be $3000.00? She needs to know that. And does that number include a salary for her and profit? From her wording, it does not. That is quite a bit of risk with no projected income or profit.
I'm actually working on the same thing right now. I have been planning a store for about six months and will probably not open until after school starts. In order to forcast, you need to determine opening foot traffic to the location and have a plan to attract the desired (or actually needed) amount. You can start by determining if your location is located where you will get traffic from other stores or are you a destination business... one where they will have to seek you out. Do you have a clientele from an ongoing business or is this a startup? In my case, I'm working on building a clientele with my commercial kitchen before I go retail so that those numbers will be more realistic. A comprehensive marketing plan is a must if the local walking traffic isn't enough.
But in high traffic areas, the rent is reflective of that built-in clientele. So both ways work.
Before and after you open, there are detailed formulas that can help determine growth. Obviously you must have a desirable product at the fair market price or nothing will work. But if the product and price is right, it comes down to closing ratios, lifetime value of a customer, cost of each person in the door, and cost of each customer who makes a purchase. The marketing plan comes in here to get x number of people in the door, x actually purchasing, retention of customers, and value of the average customer over time.
Notice I have referred several times to the "cost" of a customer... what each person through the door actually costs in money out of your pocket. Take the example of the OP. Her operating expenses are $3000.00/wk. If she gets 500 people in the door, each costs her $6.00. If out of that 500 people, only half purchase, those that purchase have to cover the cost of avg. $12.00 each. This does not include a salary for the owner or profit. This is only covering expenses. Now what if the next week only 250 people come in? Each person through the door costs her $12.00 and now each paying customer at 50% closing ratio must spend at least $24.00 average in order to just meet expenses. Next week there is a 3 foot snow storm and only 50 people come in all week, and all 50 purchase. They must purchase an average of $60.00 each because that is how much it cost her that week to see those 50 people.
Is there enough reserve capital to handle road or sidewalk construction, a parade, a detour, a storm? This will all greatly affect those above calculations.
Now let's assume this is a cupcake store. Closing ratios are probably going to be more like 80%. Let's pretend a cupcake is $3.00 and the average sale is 3 cupcakes. 418 people must come in and 334 must buy. The cost of each person through the door is $7.17.
Let's assume this is a custom cake store and the owner has a clientele that pays an average $250 per cake. In this scenario, you may get many inquiries, but you must close 12 per week just to break even.
In every one of these cases, a drop of even one sale puts the owner in the negative. Cumulative negative weeks will result in significant losses. The owner must have a reserve to cover these losses plus any income needed, as this is to be expected in the first two years.
If you know this information, you can determine what you need to increase your business and keep it on a steady growth pattern during those critical first two years with your marketing plan. It involves increasing total traffic and steadily increasing closing ratios. Next come increasing the average sale. After some time has passed, you can start looking at lifetime value of a customer (repeat business numbers). An increase in value will also increase the bottom line.
Lots to think about. But anyone looking at investing serious money in a storefront needs to know all of this forecasting in real, attainable numbers and know how to work these numbers. This is why 85% of businesses fail... lack of knowledge and lack of sufficient capital after the initial investment to keep it going.