It’s 3 months of MCT added to your account.
If it was 3x Multiple Pilot Training Certificates, then it would be advertised as that.
This thread was added on July 15, 2019, with posts from CCP_Falcon, ISD_Sakimura.
No, it just means that you’ll add the 3 months to your current game time + 90 days MCT; not sure if this will change your current subscription plan if using any other than that one (3 months subscription).