How to take into account dropout voltage of a buck converter to find minimum input voltage required?
Actually, the ~1.5V dropout voltage refers to the minimum difference between actual input and output voltage, not nominal input voltage and output voltage.  Thus you'll need at least \$ 1.5V + 3.3V = 4.8V \$ input voltage at 1A output current.

Similarly, if you want to run your circuit at 105C and draw 3A, you'll need at least \$2.2V + 3.3V = 5.5V \$.