He spent five months mourning Gwen and had stopped being Spider Man during that time. At the end, he finds the strength to be Spider Man again, but I doubt he is ready to start dating again. I think there needs to be at least a year time jump between ASM 2 and ASM 3.
Which is why using Black Cat makes so much sense.
Yes, MJ can be included as an acquaintance/possible friend, but nothing more yet. Just too soon for the audience, if not for Peter. But Black Cat, flirty and wild, going after Spiderman, not Peter, is a great "rebound girl", allowing some very fun moments, in spite of Peter not responding to Felicia's advances.
Oh, and did the Moderators really have to change the name of this thread from "The Night Gwen Stacy Died"?????