It depends entirely on what you've got with you on the surface already.

If you have a craft on the surface that can viably get into Munar orbit, then you only need to build and launch a craft from Kerbin that can a) perform a rendezvous with your first craft in Munar orbit, b) has enough space to fit your stranded kerbals, and c) and fuel to get back again.

Now, I'm kind of guessing here, but it doesn't sound like that's the scenario you're in. It sounds like you have 3 kerbals stranded on the surface who can't even get back into Mun orbit. This is trickier - firstly you need to have a reliable technique for landing close to targets on the surface. Scott Manley did a video on this once which I found to be enormously helpful - [here]() is that video.

Secondly, obviously you're going to need to have space for 3 additional kerbals in it. If you're low on the tech tree, you could try a craft that has a combination of a Mk1 command pod (1 kerbal capacity), a Mk1 passenger fuselage (2 kerbal capacity) and a probe core such as a Stayputnik, for control. The advantage of this is that you don't have to take yet more kerbals to the Mun with you for the purpose of rescuing the ones that are already there. This craft would depart Kerbin with no kerbals in it. It would also be a much lighter design. The disadvantage you'd face, though, would be that a Stayputnik doesn't provide as many sophisticated control aids as having a pilot on board. IIRC, it may not even have basic SAS equipped. It would therefore be a very tricky flight to get to your kerbals on the surface.

Alternatives would include building a bigger craft with room for 4 or 5 kerbals, and taking a crew of 1 or 2 out to the moon to rescue the 3 stuck there, or possibly using 2 smaller missions to rescue your stranded kerbals in two 'batches'. It's hard for me to say which is the best option out of those, really, without knowing more specifics about your particular game and so on.

Nonetheless, those are some suggestions :)