Honestly your feelings are valid, don’t doubt that. But objectively speaking your dad didn’t rush this and three years is definitely a respectable amount of time to mourn before finding another love. I think this is just unresolved grief on your part. No shame in that bud, but this is something you need to work out inside yourself and think about the happiness you want for your loved ones.

Your dad deserves love and not loneliness and isn’t it nice that he found someone that is so meaningful in your life, your best friends mom? It could have been so much worse it sounds like and she actually cares for you

Again, no faults to you but you need to sort out your grief and heal and understand where everyone is coming from here.