Dear All

I have a Stata dataset with 200 observations. Each observation represents 1 order. The dataset contains a variable indicating the month of the order (January, February or March).
It is also known how much was spent on advertising in those 3 months. Unfortunately I only have information from these 3 months. Currently, this information is contained in 3 different variables that have a constant value. For example, the variable AdvertisingJanuary has a value of 10000 for all 200 observations. This advertising could also have an impact on sales in the coming months.
I would like to estimate the turnover by taking into account the advertising, among other things, but I don't know how to process these variables in the model. After all, including a constant variable in the model results in perfect multicollinearity.

Thanks in advance.