Forecasting is a bit of an art. You need to build a model of your sales. Are the sales seasonal? Do you need to worry about recession? What is your supply chain like? Can you expect to have as many units to sell as demand requires? Will advertising affect things?
With the extremely limited information you provided, about all you can do is say that sales for 3 months are 12420+1200+12400=26000. Average monthly sales is 26000/3=8,666. So estimated sales for the next 12 months is 8,666*12=104,000. But that's really suspect. Your June amount is a LOT lower than the others. Is that a typo, or is there a reason it's so low? If there's a reason, can you incorporate that reason into your model?