A is a stronger player than B so probability of wining A is higher then probability of wining B .
let W=win L=lost
Probability of wining A to you = 3/4 , W= 0.75 L=0.25
Probability of wining B to you = 1/2=0.50 L=0.5
Case 1: AAB
probability for you to win series = WWL + LWW+WWW= 0.25*0.25*0.5+0.75*0.25*0.5+0.25*0.25*0.5=0.15625
Case 2: ABA
probability for you to win series = WWL + LWW+WWW= 0.25*0.5*0.75+0.75*0.5*0.25+0.25*0.5*0.25=0.21875
Case 3: BAB
probability for you to win series = WWL + LWW+WWW=0.5*0.25*0.5+0.5*0.25*0.5+0.5*0.25*0.5=0.1875
Case 4: BAA
probability for you to win series = WWL + LWW+WWW=0.15625
so ABA sequence will maximize our chances to win seriese Ans=2