Hi Ashsantos, I agree with a lot of what the other posters have said. This is a spectacular cake for a 3-year old's birthday!
I would let the customer know that a single-tier is the standard order for a 3 year old's birthday cake, but if she would like a three-tier then this is obviously going to involve three times the ingredients and three times the labour.
When I started out in the business, I had set prices from day one and this avoided so many awkward conversations with customers expecting the sun, moon & stars for little or nothing in terms of payment.
As for the fact that you're a new decorator, this is irrelevant once you are able to make and decorate the cakes your customers request. So there's no need to charge less than your counterparts in your area. In fact, it will make it harder in the long run to get your prices up. In any case, custom cakes are a luxury product. If you charge less than the going rate, people will infer inferior quality.
It's tough when you're starting out to have the confidence to deal with customers' put downs over prices, but you have to stand your ground and be willing to lose the order than cut your rates. Otherwise you will be working for much less than the minimum wage catering to customers whims, when your time would be better off spent with your family and friends.
Best of luck!