How to deal with missing variables in a Linear Regression

I want to run a LR to check the relationship between those 3 independent variables on a dependent variable(Not a time series), can i include the third variable from the start of the period? Or I should start from the date the variable is not null ?

https://preview.redd.it/4crz0asf4xkb1.png?width=407&format=png&auto=webp&s=2fff1c005fa9a84d88f6b21acff3c5754efe481f

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