To estimate the total market size for opening an ice cream shop, we need to consider several factors including target audience, competition, industry trends, and market capacity. Here is a step-by-step process to derive the total market size:

1. Identify the target audience: Determine the specific demographic and psychographic characteristics of the target audience for the ice cream shop. This could include factors such as age, gender, income level, and preferences.
2. Analyze the competition: Research and evaluate the existing ice cream shops in the target market. Consider factors such as their market share, pricing strategies, product offerings, and customer satisfaction levels. This will help understand the level of competition and market saturation.
3. Study industry trends: Examine the current and projected trends in the ice cream industry. This could include factors such as increasing consumer demand for artisanal or organic ice cream, the popularity of unique flavors, or the rise of vegan or lactose-free options. Understanding these trends will help estimate the potential growth of the market.
4. Determine the market capacity: Calculate the total market capacity by considering the population size of the target market and the average consumption rate of ice cream per person. For example, if the target market has a population of 100,000 and the average consumption rate is 10 ice creams per person per year, the market capacity would be 1,000,000 ice creams.
5. Estimate market share: Based on the analysis of competition and industry trends, make an educated estimate of the potential market share the ice cream shop could capture. This could be a percentage of the total market capacity. For example, if the ice cream shop is expected to capture 10% of the market share, the estimated market size would be 100,000 ice creams.
6. Project for three years: Consider the growth potential of the ice cream shop over a three-year period. This could involve analyzing factors such as the expected increase in customer base, expansion plans, and marketing strategies. Adjust the estimated market size accordingly for each year based on these projections.

By following this process and using relevant data and analysis, you can derive an estimate of the total market size for opening an ice cream shop over the next three years.