Eczema (Atopic Dermatitis): This is a common condition in children that can cause redness and blistering, especially in areas like the anus due to irritation from feces. The age of the child and the description of the symptoms align well with eczema.

Contact Dermatitis: This could be due to an allergic reaction or irritation from diapers, wipes, or soaps, leading to redness and blisters around the anus.
Impetigo: A contagious skin infection that can cause red sores or blisters, which can appear anywhere on the body, including the anal area.
Diaper Rash (Irritant Dermatitis): Prolonged exposure to a soiled diaper can cause irritation and lead to redness and blisters around the anus.

Child Abuse: Although less common, it's crucial to consider the possibility of physical abuse, which could manifest as unexplained injuries, including blisters or redness around the anus.
Sexually Transmitted Infections (STIs): In rare cases, STIs like genital herpes could present with blisters in the anal area, and it's essential to consider this possibility, especially if other signs or risk factors are present.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease or ulcerative colitis can cause perianal disease, including skin tags, fistulas, or abscesses, which might initially be mistaken for blisters.

Zinc Deficiency: Can cause skin lesions and might lead to diaper dermatitis or perianal dermatitis.
Staphylococcal Scalded Skin Syndrome (SSSS): A rare condition caused by staphylococcal toxins that can lead to widespread blistering, including potentially around the anus.
Autoimmune Disorders: Certain autoimmune conditions, like pemphigus, could theoretically cause blistering skin lesions, although these would be extremely rare in a three-year-old and would likely involve other areas of the body as well.