The suspense created in Act 3 comes from the fact that the trials have started, and the people are in a full-blown fervor.
The suspense is high, because characters we care about are beginning to be involved heavily in the trials themselves. Consider Danforth’s interrogation of Proctor.
I understand well, a husband's tenderness may drive him to extravagance in defense of a wife. Are you certain, Mister, that your evidence is the truth? (Act 3)
Danforth also comments that since Satan is involved, this is no ordinary crime. He says that “witchcraft by its nature is an invisible crime” (Act 3). At this point we are beginning to realize that Danforth just wants to hang as many people as possible. He is not trying to get to the actual truth. This will cause fear and suspense in the reader, because the reader will realize that the laws of reality and common sense do not exist here.