Phi[al_,be_]=Gamma[1-al] Gamma[1-be]/Gamma[1-al-be];

als=a0+ a1 s; alt= a0+ a1 t; alu= a0 + a1 u; (*a0= 1/2 -al1 mpisq;*)

A=-1/2(Phi[(als-1/2) mpisq+1/2,(alt-1/2) mpisq+1/2]+Phi[(als-1/2) mpisq+1/2, (alu-1/2) mpisq+1/2]-Phi[(alt-1/2) mpisq+1/2, (alu-1/2) mpisq+1/2]) ;

Series[A,{mpisq,0,1}] // Normal // Simplify