Is she bred to the same stallion, or a different one from the prior 2 foals? If a different one, then depending on his progeny, you may have a good chance at a filly!!
Some stallions just throw more 'y' chromosomes. I know when selecting a stallion here, I always looks to see his percentages of fillies to colts. I always chose the boys with high filly percentages, and was never disappointed.
Of course, sometimes you just have to HAVE a stallion no matter what his production record! LOL