Breaking off a collaboration/project in science
I assume that the two other students were not "forced" on your project by some higher authority, so the call on whether or not to collaborate is yours to make.

If that is the case, you should as soon as possible sit down with the other two students and tell them that you are not going to work with them any longer. The major point here is be civil and professional. If their contribution is large enough, you can put them as co-authors or in the acknowledgements section. I say as soon as possible, because 3 weeks is not that much of time for them to get too seriously involved. For the purpose of other people, all three of you should assume a neutral stand point, i.e. something along the lines of "things didn't work out" without going into extensive details. Although, you should definitely bring your adviser up-to-date the real reasons for stopping the collaboration. It is never good to wash your dirty laundry in public, so badmouthing on either side just hurts all the involved parties (one reason more to keep the adviser in the loop).

Be adamant about your decision, as I guess that your colleagues and the adviser will try to smooth things out one way or the other (asking you to reconsider, for some more time, increasing their contribution, etc).