Divide students in groups based on preferences

I am writing an application using C#/Asp.Net. In the database i have three tables. (Students, Groups and Preference).

Students

Student Name---------Group ID(Not Data for now)

Student A

Student B

Student C

Student D

Student E

Student F

Student G

Student H

Student I

Student J

Student K

Student L

Preferences Tables

Student Name--------Preference 1--------Preference 2--------Preference 3

Student A------------Student G-----------Student F

Student B------------Student H-----------Student K------------Student L

Student C------------Student I

Student D

Student E

Student F

Student G

Student H

Student I

Student J

Student K

Student L

Groups Table

Group ID

1

2

3

I want to create students groups for Labs based on preferences. Each group will have min 3 and max 4 students. If some students are left without group, that can be adjusted manually. Each student will enter max 3 preferences (optional), who he/she want's to group with. How can i implement this. Any Examples etc? Any help will be much appreciated.