If you wanted to brute force it, you could use Robert's formula three times as the Period field:
=GLActual(blahblah,ROUNDUP(MONTH(A1)/3,0)\*3,blahblah)+GLActual(blahblah,(ROUNDUP(MONTH(A1)/3,0)\*3)+1,blahblah)+GLActual(blahblah,(ROUNDUP(MONTH(A1)/3,0)\*3)+2,blahblah)