SELECT
`Distinct1`.`UserId`,
`Distinct1`.`FirstName`,
`Distinct1`.`LastName`,
`Distinct1`.`EmailAddress`,

FROM ( SELECT DISTINCT
`Extent2`.`UserId`,
`Extent2`.`FirstName`,
`Extent2`.`LastName`,
`Extent2`.`EmailAddress`,

    FROM `AssistantTo` AS `Extent1`
    INNER JOIN `User` AS `Extent2` ON `Extent1`.`AssistantId` = `Extent2`.`UserId`
    INNER JOIN `CustomTagUser` as `Extent3` ON `Extent3`.`UserId` = `Extent2`.`UserId`
 WHERE `Extent1`.`OwnerId` = 274 AND `Extent3`.`CustomTagId` = 114
 ) AS `Distinct1`

var assistants =
                    dbContext.AssistantsTo
                        .Include(x => x.Assistant)
                        .Include(x => x.Assistant.CustomTagUser)
                        .Where(at =>
                            at.OwnerId == currentUser.UserId
                            && (at.Assistant.CustomTagUser.Count(y => y.CustomTagId == filter) > 0)
                            )
                        .Select(at => at.Assistant)
                        .Distinct()
                        .ToList();

`Extent3`.`CustomTagId` = 114

public class AssistantTo
    {
        [Key]
        [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
        public int AssistantToId { get; set; }
        public int AssistantId { get; set; }
        [ForeignKey("AssistantId")]
        public virtual User Assistant { get; set; }
        public int OwnerId { get; set; }
        [ForeignKey("OwnerId")]
        public virtual User Owner { get; set; }
    }

    public class CustomTagUser
    {
        [Key]
        public int CustomTagUserId { get; set; }
        public int CustomTagId { get; set; }
        public int UserId { get; set; }
        public DateTime CreatedOn { get; set; }
        [ForeignKey("CustomTagId")]
        public virtual CustomTags CustomTags { get; set; }
        [ForeignKey("UserId")]
        public virtual User User { get; set; }
    }


[Table("User")]
    public class User
    {
        [Key]
        [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
        public int UserId { get; set; }

        public string FirstName { get; set; }
        public string LastName { get; set; }
        public string EmailAddress { get; set; }
        public string Password { get; set; }
        .....
        public ICollection<CustomTagUser> CustomTagUser { get; set; }
    }
}