diff --git a/task3/main.go b/task3/main.go
index 1d398f6..c5708e3 100644
--- a/task3/main.go
+++ b/task3/main.go
@@ -1,11 +1,161 @@
 package main
 
 import (
+	"encoding/json"
+	"errors"
+	"flag"
 	"io"
+	"io/ioutil"
 	"os"
 )
 
-func Perform(args Arguments, writer io.Writer) error {
+type Arguments map[string]string
+
+type User struct {
+	Id    string `json:"id"`
+	Email string `json:"email"`
+	Age   int    `json:"age"`
+}
+
+func filterUsers(users []User, filter func(user User) bool) (result []User) {
+	for _, user := range users {
+		if filter(user) {
+			result = append(result, user)
+		}
+	}
+	return
+}
+
+func readCollection(fileName string) (result []User, err error) {
+	text, err := ioutil.ReadFile(fileName)
+	if err != nil {
+		if os.IsNotExist(err) {
+			//suppress this warning, if file is not presented then collection is empty
+			return result, nil
+		}
+		return
+	}
+	err = json.Unmarshal(text, &result)
+	return
+}
+
+func write(users []User, fileName string) (err error) {
+	content, err := json.Marshal(users)
+	if err != nil {
+		return
+	}
+	return ioutil.WriteFile(fileName, content, 777)
+}
+
+var operations = map[string]func(Arguments, io.Writer) error{
+	"list": func(arguments Arguments, writer io.Writer) (err error) {
+		data, err := ioutil.ReadFile(arguments["fileName"])
+		if err != nil {
+			return
+		}
+		_, err = writer.Write(data)
+		return
+	},
+	"remove": func(arguments Arguments, writer io.Writer) (err error) {
+		if len(arguments["id"]) == 0 {
+			return errors.New("-id flag has to be specified")
+		}
+
+		users, err := readCollection(arguments["fileName"])
+		if err != nil {
+			return
+		}
+
+		result := filterUsers(users, func(user User) bool {
+			return user.Id != arguments["id"]
+		})
+
+		if len(users) == len(result) {
+			_, err = writer.Write([]byte("Item with id " + arguments["id"] + " not found"))
+			return
+		}
+
+		return write(result, arguments["fileName"])
+	},
+	"add": func(arguments Arguments, writer io.Writer) (err error) {
+		if len(arguments["item"]) == 0 {
+			return errors.New("-item flag has to be specified")
+		}
+
+		var userToAdd User
+		if err = json.Unmarshal([]byte(arguments["item"]), &userToAdd); err != nil {
+			return
+		}
+
+		users, err := readCollection(arguments["fileName"])
+		if err != nil {
+			return
+		}
+
+		var usersWithSamePK = filterUsers(users, func(user User) bool {
+			return user.Id == userToAdd.Id
+		})
+
+		if len(usersWithSamePK) != 0 {
+			_, err = writer.Write([]byte("Item with id " + userToAdd.Id + " already exists"))
+			return
+		}
+
+		return write(append(users, userToAdd), arguments["fileName"])
+	},
+	"findById": func(arguments Arguments, writer io.Writer) (err error) {
+		if len(arguments["id"]) == 0 {
+			return errors.New("-id flag has to be specified")
+		}
+
+		users, err := readCollection(arguments["fileName"])
+		if err != nil {
+			return
+		}
+
+		var filteredUsers = filterUsers(users, func(user User) bool {
+			return user.Id == arguments["id"]
+		})
+
+		if len(filteredUsers) == 0 {
+			_, err = writer.Write([]byte(""))
+			return
+		}
+
+		content, err := json.Marshal(filteredUsers[0])
+		if err != nil {
+			return
+		}
+
+		_, err = writer.Write(content)
+		return
+
+	},
+}
+
+func Perform(arguments Arguments, writer io.Writer) (err error) {
+	if len(arguments["operation"]) == 0 {
+		return errors.New("-operation flag has to be specified")
+	}
+	if len(arguments["fileName"]) == 0 {
+		return errors.New("-fileName flag has to be specified")
+	}
+
+	operation, operationExists := operations[arguments["operation"]]
+	if !operationExists {
+		return errors.New("Operation " + arguments["operation"] + " not allowed!")
+	}
+
+	return operation(arguments, writer)
+}
+
+func parseArgs() Arguments {
+	operation := flag.String("operation", "", "Specify operation")
+	id := flag.String("id", "", "Specify id")
+	item := flag.String("item", "", "Specify item")
+	fileName := flag.String("fileName", "", "Specify fileName")
+	flag.Parse()
+	return Arguments{"operation": *operation, "id": *id, "item": *item, "fileName": *fileName}
 }
 
 func main() {