There are seven possible combinations
However, if the hats were [r b b], 1 would know her hat instantly, and if the hats were [b r b], 2 would know her hat instantly. That leaves five possible combinations. Also, this means that if she sees one blue and one red, she knows her hat must be red. And, obviously, if she sees two blue hats she knows her hat is red.
There are five possible combinations, and three of them work out to 3's hat being red. The two tricky ones left over are [r r r] and [r r b].
However, if the hats were [r r b], 2 would see 3's blue hat and know her own hat can't possibly be blue, or else 1 would answer red. [R r b] gets thrown out.
In all the cases where the first two answers are don't know, 3's hat must be red.