Question

calculate probability of obtaining a head in first toss and tail in second toss as well as third toss of the coins

Answer 1

Any subsequent tosses do not impact the probability of the first flip. We can examine a tree diagram to see that this is true.

There are four equally likely outcomes; HH, HT, TH, and TT. Of these 4 outcomes, two have a head as the result of the first toss. 2/4 = 1/2 = 50%.