The earning period runs for three months. How do I create a formula to result in date of the last day of month?

I have an 'Earning Period Start' date field and an 'Earning Period End' formula-date field. The earning period runs for three months. How do I create a formula for a three month earning period that results in the last day of the third month? (i.e. Earning Period Starts 1/1/2019, Earning Period ends 3/31/2019). This is the formula I started with but it gives me the date of the first day of the third month, not the last day: 

=AdjustMonth([Earning Period Start],3)