public class MyModel : IValidatableObject
    {
        [Key]
        public int Id { get; set; }

        public int Item1 { get; set; }

        public int Item2 { get; set; }

        public int Item3 { get; set; }

        public IEnumerable<ValidationResult> Validate(ValidationContext validationContext)
        {
            if (Item3 < Item1 + Item2)
            {
                yield return new ValidationResult("Item1 + Item2 must be less than Item3", new List<string> { "Item3" });
            }
        }
    }