I have Difference-in-Differences models with 3 periods: pre-treatment, treatment and post-treatment periods.
Normally, we could do DID in the following way: run DID between pre-treatment and treatment (DID_1 shown in the pic); or run the DID between treatment and post-treatment period (DID_2).
I think both of the methods could test the treated effects. DID_2 could be considered as a robustness check.
Is there any way to run this DID with these 3 periods together? (like run one regression, but not two separate regressions?) If these two methods have different conclusions, which one should be correct? Thanks.