Forecasting with two separate periods of time

I have a sales forecasting problem where I only have three months of year 2018 ( Jan, fev, march) and three months of year 2019 (oct, nov,dec) and I have to forecast the sales for all the months of 2020.
I am new with the forecasting problems and what I can conclude from these data that it is time series data but it’s so limited and the gap between the two years is about 18 months so handling missing data won’t be a solution for this. I have been so stack can anyone help me please.
Can you suggest me what to do ?