More data than you ever wanted to see on the immune response to the Pfizer 3rd dose can be seen here:
This is the data that Pfizer presented to the FDA advisory committee. Perhaps the most telling are Figures 2 and 3 (pages 19 & 20) that show that the amount of anti-virus antibodies produced by patients receiving a 3rd dose was significantly higher (the tall collumns) than those receiving only two doses (the shorter columns).